Method of using a crystal of the N-terminal domain of a signal transducer and activator of transcription

ABSTRACT

The present invention provides a crystal containing the N-terminal domain of a STAT protein that is of sufficient quality to perform X-ray crystallographic studies. Methods of preparing the crystals are include in the invention. The present invention further discloses the three-dimensional structure of the crystal. The present invention also provides methods of using the structural information in drug discovery and drug development.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a Divisional Application of U.S. Ser. No. 09/012,710 filed Jan. 23, 1998, now U.S. Pat. No. 6,087,478, the disclosure of which is hereby incorporated by reference in its entirety. Applicants claim the benefit of this application under 35 U.S.C. §120.

GOVERNMENTAL SUPPORT

The research leading to the present invention was supported, at least in part, by NIH Grant Nos. AI32489 and AI34420. Accordingly, the Government may have certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to structural studies of STAT proteins, modified STAT proteins and more particularly the N-terminal domain of STAT proteins. Included in the present invention is a crystal of the N-terminal domain of a STAT protein and corresponding structural information obtained by X-ray crystallography. The present invention also relates to methods of using the crystal and related structural information in drug screening assays.

BACKGROUND OF THE INVENTION

Transcription factors play a major role in cellular function by inducing the transcription of specific mRNAs. Transcription factors, in turn, are controlled by distinct signaling molecules. The STATs (Signal Transducer and Activator of Transcription) constitute a family of transcription factors necessary to activate distinct sets of target genes in response to cytokines and growth factors [Darnell et al. WO 95/08629, (1995)]. The STAT proteins are activated in the cytoplasm by phosphorylation on a single tyrosine residue [Darnell et al., Science 264:1415 (1994)]. The responsible kinases are either ligand-activated transmembrane receptors with intrinsic tyrosine kinase activity, such as EGF- or PDGF-receptors, or cytokine receptors that lack intrinsic kinase activity but have associated JAK kinases, such as those for interferons and interleukins [Ihle, Nature 377:591-594 (1995)]. One distinctive characteristic of the STAT proteins are their apparent lack of requirement for changes in second passenger, e.g., cAMP or Ca⁺⁺, concentrations. Presently, there are seven known mammalian STAT family members. The recent discovery of a Drosophila STAT protein, suggests that these proteins have played an important role in signal transduction since the early stages of our evolution [Darnell, PNAS, 94:11767-11769 (1997)].

Each STAT protein contains a SRC homology domain (SH2 domain). When activated, the STAT proteins are phosphorylated, and form homo- or heterodimeric structures in which the phosphotyrosine of one partner binds to the SH2 domain of the other. The reciprocal SH2-phosphotyrosine interactions between two STAT proteins result in the formation of an active dimer that translocates to the nucleus and activates specific gene expression [Darnell et al., Science 264:1415 (1994)] by binding to a canonical recognition site for the STAT dimer. This canonical recognition site encompasses 9-10 base pairs (TTCN₃₋₄GAA) of DNA [Horvath et al., Genes & Devel. 9:984 (1995); Seidel et al., Proc. Natl. Acad. Sci. USA 92:3041 (1995); Ihle, Cell 84:331 (1996); Mikita et al., Mol. Cell. Biol. 16:5811 (1996)]. Analysis of the binding of activated STATs to DNA targets has revealed that the STAT binding sites can extend over two or more adjacent canonical sites [Xu et al., Science 273:750 (1996); Meier and Groner, Mol. Cell. Biol. 14:128 (1994); Symes et al., Molecular Endocrinology 8;1750 (1994); Dajee et al., Molecular Endocrinology 10:171 (1996); John et al., EMBO J. 15:5627 (1996)].

STAT proteins serve in the capacity as a direct messengers between the cytokine or growth factor receptor present on the cell surface, and the cell nucleus. However, since each cytokine and growth factor produce a specific cellular effect by activating a distinct set of genes, the means in which such a limited number of STAT proteins mediate this result remains a mystery. Indeed, at least twenty-five different ligand-receptor complexes signal the nucleus through the seven known mammalian STAT proteins [Yan et al., Cell 84:421-430 (1996)].

There is increasing evidence that mammalian transcription factors activate transcription and achieve biological specificity by interactions with other transcription factors, trans-activators or the general transcription machinery [McKnight, Genes & Development 10:367 (1996); Roeder, Trends in Biochemical Sciences 21:327 (1996)]. Although the molecular basis for these phenomena is poorly understood, direct protein:protein interactions among multiple promoter bound proteins appear to mediate this synergistic activation [Tijan and T. Maniatis, Cell 77:5 (1994)].

In the case of the STATs, a small N-terminal domain has been shown to mediate a number of important protein:protein interactions that influence transcriptional outcome [Leung et al., Science 273:750 (1996); Vinkemeier et al., EMBO J. 15: 5616 (1996)]. This domain allows cooperative interactions between STAT dimers bound to adjacent target sites on DNA, leading to a drastically prolonged half-life of the protein-DNA complex [Vinkemeier et al., EMBO J. 15: 5616 (1996)]. Functional assays exploring the induction of the hepatic Spi 2.1 gene revealed the necessity for cooperative STAT binding to two adjacent recognition sites for a full growth hormone response [Bergad et al., J. Biol. Chem. 270, 24903 (1995)]. In addition, it was observed that these cooperative contacts affect the binding site selection of different STATs on a natural promoter that contains multiple potential STAT recognition sites [Xu et al., Science 273:750 (1996)]. Each of the oligomerized STAT-1, -4, and -5 dimers were shown to bind to a different combination of canonical sites [Xu et al., Science 273:750 (1996)]. Deletion of the N-terminal ˜100 residues of STAT-1 and STAT-4 abolishes cooperative binding to DNA [Xu et al., Science 273:750 (1996); Vinkemeier et al., EMBO J. 15: 5616 (1996)]. The truncated protein fully retains binding to a single target site as a dimer, suggesting that the N-terminal domain is dispensable for dimer formation and DNA binding [Xu et al., Science 273:750 (1996); Vinkemeier et al., EMBO J. 15: 5616 (1996)], but is necessary for interaction between STAT dimers and binding site discrimination [Xu et al., Science 273:750 (1996)]. Also, the N-domain of STAT-1 is required for interaction between STAT-1 and the transcriptional co-activator protein CBP, a large (˜2500 amino acids) polypeptide with transacetylase activity [Zhang et al., Proc. Natl. Acad. Sci. USA 93:15092 (1996)]. Additionally, the amino-terminal region of STAT-2 is involved in binding to the intracellular region of the interferon-α receptor [Leung et al., Mol. Cell. Biol. 15:1312 (1995)].

Therefore, there is a need to obtain agonists and antagonists that can modulate the effect of STAT proteins during specific gene activation. In particular, there is a need to obtain drugs that will directly interact with the important N-terminal domain of STAT proteins. Unfortunately, identification of such drugs have heretofore relied on serendipity and/or systematic screening of large numbers of natural and synthetic compounds. A far superior method of drug-screening relies on structure based drug design. In this case, the three dimensional structure of a protein or protein fragment is determined and potential agonists and/or potential antagonists are designed with the aid of computer modeling [Bugg et al., Scientific American, Dec.:92-98 (1993); West et al., TIPS, 16:67-74 (1995)]. However, heretofore the three-dimensional structure of a STAT protein or fragment thereof has remained unknown, essentially because no such protein crystals had been produced of sufficient quality to allow the required X-ray crystallographic data to be obtained.

Therefore, there is presently a need for obtaining an N-terminal STAT domain fragment that can be crystallized to form a crystal with sufficient quality to allow such crystallographic data to be obtained. Further, there is a need for such crystals. Furthermore there is a need for the determination of the three-dimensional structure of such crystals. Finally, there is a need for procedures for related structural based drug design based on such crystallographic data.

The citation of any reference herein should not be construed as an admission that such reference is available as “Prior Art” to the instant application.

SUMMARY OF THE INVENTION

The present invention provides a crystal containing the N-terminal domain of a STAT protein which effectively diffracts X-rays and thereby allows the determination of the atomic coordinates of the N-terminal domain to a resolution of greater than 5.0 Angstroms. In a preferred embodiment of this type the crystal effectively diffracts X-rays for the determination of the atomic coordinates of the N-terminus to a resolution of greater than 3.0 Angstroms. In a more preferred embodiment of this type the crystal effectively diffracts X-rays for the determination of the atomic coordinates of the N-terminus to a resolution of greater than 2.0 Angstroms.

In one embodiment the N-terminal domain of the crystal comprises the amino acid sequence of:

Arg Xaa ^(H)Xaa Leu Xaa Xaa Trp ^(H)Xaa Glu Xaa Gln Xaa Trp (SEQ ID NO:1), where ^(H)Xaa can be either Ile, Leu, Val, Phe, or Tyr and Xaa can be any amino acid. In another embodiment the crystal of the N-terminal domain of the STAT protein is contained in a STAT fragment that consists of 100 to 150 amino acids. In a preferred embodiment the STAT fragment comprises amino acids 4-112 of SEQ ID NO:9. In a more preferred embodiment the crystal contains an N-terminal domain of a STAT protein comprising amino acid residues 2-123 of SEQ ID NO:9 with 5 additional amino acid residues N-terminal to amino acid residue number 2, i.e., from the N-terminus GLY SER GLY GLY GLY (SEQ ID NO:3), amino acid residue 2. In one embodiment of this type the crystal effectively diffracts X-rays to allow the determination of the atomic coordinates of the N-terminus to a resolution of 1.45 Angstroms.

The present invention provides a crystal of the N-terminal domain having a space group of P6₅22 and a unit cell of dimensions a=79.51 Å, b=79.51 Å, and c=84.68 Å. The present invention further provides a crystal of the N-terminal domain having secondary structural elements comprising eight helices (α1-α8) that are assembled into a hook-like structure that has an inner and outer surface. The first four helices (α1-α4) form a ring-shaped element having a proximal and a distal surface, whereas helices six (α6) and seven (α7) form an antiparallel coiled-coil that also has a proximal and a distal surface. Helix five (α5) connects the ring-shaped element to the anti-parallel coiled-coil, while helix eight (α8) is wrapped around the distal surface of the ring-shaped element. The inner surface of the hook-like structure is formed by the intersection of the proximal surface of the ring-shaped element with the proximal surface of the antiparallel coiled-coil.

The present invention also provides a method of growing a crystal of an N-terminal domain of a STAT protein. The method of making the crystal comprises placing an aliquot of a solution containing a STAT N-terminal domain fragment of the present invention on a cover slip as a hanging drop above a well containing a reservoir buffer that comprises 0.2 M Na⁺CH₃COO⁻, 0.1 M Tris/HCL pH 8.0, 17% PEG4000. All of the STAT N-terminal domain fragments of the present invention may be used in this manner to prepare such a crystal. In one specific embodiment the solution containing a STAT N-terminal domain fragment is prepared by combining a preparation of the STAT N-terminal domain fragment that contains 20 mg/ml of the STAT N-terminal domain fragment in 50 mM Hepes/HCl pH 8.0, 150 mM KCl, 2.5 mM CaCl₂, and 5 mM DTT with an equal volume of the reservoir buffer. In a preferred embodiment the aliquot comprises approximately 1 μl of the solution. In another preferred embodiment the STAT N-terminal domain fragment comprises the amino acid sequence of:

Arg Xaa ^(H)Xaa Leu Xaa Xaa Trp ^(H)Xaa Glu Xaa Gln Xaa Trp (SEQ ID NO:1), where ^(H)Xaa can be either Ile, Leu, Val, Phe, or Tyr. In a more preferred embodiment the N-terminal domain is a STAT-4 N-terminal domain fragment. In a still more preferred embodiment the STAT-4 N-terminal domain fragment contains amino acid residues 2-123 of SEQ ID NO:9 with 5 additional amino acid residues N-terminal to amino acid residue number 2, i.e., from the N-terminus GLY SER GLY GLY GLY, amino acid residue 2.

The present invention also provides methods of screening drugs that either enhance or inhibit STAT-STAT dimeric interactions. Such methods include those that identify drugs that effect the interaction of N-terminal domains of STAT proteins that are bound to adjacent DNA binding sites. In one such embodiment, a drug library is screened by assaying the binding activity of a STAT protein to its DNA binding site. This assay is based on the ability of the N-terminal domain of STAT proteins to substantially enhance the binding affinity of two adjacent STAT dimers to a pair of closely aligned DNA binding sites, i.e., binding sites separated by approximately 10 to 15 base pairs. Such drug libraries include phage libraries as described below, chemical libraries compiled by the major drug manufacturers, mixed libraries, and the like. Any of such compounds contained in the drug libraries are suitable for testing as a prospective drug in the assays described below, including in a high throughput assay based on the methods described below.

An antagonist of the STAT N-terminal dimeric interaction antagonizes one or more aspects of the binding of the STAT dimers to adjacent weak binding sites for the STAT dimers on a promoter of a gene. Such antagonists could be useful as drugs in the treatment of a variety of disease states, including inflammation, allergy, asthma, and leukemias.

On the other hand, a drug that acts as an agonist stabilizes the N-terminal dimeric interaction between STAT dimers bound to adjacent weak binding sites for the STAT dimers on a promoter of a gene, thereby enhancing STAT function. Such agonists can be used as drugs that are useful in the treatment of anemias, neutropenias, thrombocytopenia, cancer, obesity, viral diseases and growth retardation, or other diseases characterized by an insufficient STAT activity.

Therefore the present invention provides a method of using the crystals of the present invention in drug screening assays. In one such embodiment the method comprises selecting a potential drug by performing rational drug design with the three-dimensional structure determined for the crystal. The selecting is preferably performed in conjunction with computer modeling. The potential drug is contacted with a dimeric STAT protein N-terminal domain and the binding of the potential drug with the N-terminal domain is detected. A drug is selected which binds to the N-terminal domain of the dimeric STAT protein.

In a preferred embodiment of this type, contacting the potential drug with a dimeric STAT protein N-terminal domain is performed with dimeric STAT protein fragments containing the STAT protein N-terminal domain. In a more preferred embodiment the STAT protein N-terminal domain has an amino acid sequence comprising SEQ D NO:1. In one such embodiment the dimeric STAT protein fragment is labeled. In another such embodiment the dimeric STAT protein fragment is bound to a solid support.

Another method of using a crystal of the present invention in a drug screening assay comprises selecting a potential drug by performing rational drug design with the three-dimensional structure determined for the crystal. The selecting is preferably performed in conjunction with computer modeling. The potential drug is contacted with two or more dimeric STAT proteins in the presence of a nucleic acid containing at least two adjacent weak binding sites for STAT protein dimers and the effect of the potential drug on the binding of the dimeric STAT proteins to each other and/or to the nucleic acid is detected. A potential drug is selected as a candidate drug when it either enhances or diminishes the binding of the dimeric STAT proteins to each other and/or the nucleic acid. In a preferred embodiment of this type the method further comprises growing a supplemental crystal containing a protein-drug complex formed between the dimeric N-terminal domain of the STAT protein and the candidate drug. A crystal is chosen that effectively diffracts X-rays allowing the determination of the atomic coordinates of the protein-ligand complex to a resolution of greater than 5.0 Angstroms, preferably to a resolution greater than 3.0 Angstroms and more preferably to a resolution greater than 2.0 Angstroms. The three-dimensional structure of the supplemental crystal is determined by molecular replacement analysis and a drug is selected by performing rational drug design with the three-dimensional structure determined for the supplemental crystal. The selecting is preferably performed in conjunction with computer modeling.

The present invention also provides a method for identifying a drug that modulates the ability of adjacent STAT protein dimers to interact and bind to adjacent DNA binding sites. One such embodiment comprises selecting a potential drug by performing rational drug design with the three-dimensional structure determined for a crystal of the present invention. The selecting is preferably performed in conjunction with computer modeling. The binding affinity of the STAT protein (or of a fragment thereof that comprises the N-terminal domain) for a nucleic acid comprising two adjacent weak STAT DNA binding sites in the presence and absence of the potential drug is determined (i.e., measured). The binding affinity of the STAT protein (or the fragment) for a nucleic acid comprising a single strong STAT binding site in the presence and absence of the potential drug is also determined. Next a comparison is made between the binding affinities of the STAT protein (or the fragment) is measured for the two adjacent weak STAT DNA binding sites in the presence and absence of the potential drug with that determined for the STAT protein (or the fragment) for the single strong STAT binding site in the presence and, absence of the potential drug. A potential drug which causes an increase in the binding affinity measured for the two adjacent weak STAT DNA binding sites but not in the binding affinity measured for the single strong STAT binding site is identified as a drug that enhances the interaction between adjacent activated STAT dimers. On the other hand, a potential drug which causes a decrease in the binding affinity measured for the two adjacent weak STAT DNA binding sites but not in the binding affinity measured for the single strong STAT binding site is identified as a drug that inhibits the interaction between adjacent activated STAT dimers.

The present invention further provides a method for identifying a drug that modulates the ability of adjacent STAT protein dimers to interact and bind to adjacent DNA binding sites which comprises measuring the binding affinity of the STAT protein comprising a functional N-terminal domain to a nucleic acid comprising two adjacent weak STAT DNA binding sites w in the presence and absence of a potential drug. The binding affinity of a modified form of the STAT protein that lacks a functional N-terminal domain is also determined for the nucleic acid in the presence and absence of the potential drug. The binding affinity measured for the STAT protein in the presence and absence of the potential drug is then compared with the binding affinity measured for the modified STAT protein in the presence and absence of the potential drug. A potential drug which causes an increase in the binding affinity measured for the functional STAT protein but not in the binding affinity measured for the modified STAT protein is identified as a drug that enhances the interaction between adjacent activated STAT dimers, and a potential drug which causes a decrease in the binding affinity measured for the STAT protein but not in the binding affinity measured for the modified STAT protein is identified as a drug that inhibits the interaction between adjacent activated STAT dimers. Variations of these assays are performed in in situ assays as described below with reporter genes operably under the control of weak and/or strong STAT binding sites.

In one such embodiment the modified STAT protein is a STAT protein that lacks the α4-tryptophan. In another such embodiment the modified Stat protein lacks the α4-glutamic acid. In still another embodiment the modified STAT protein is the truncated STAT protein Stat1to identified in by Vinkemeier et al.[EMBO J. 15:5616 (1996)].

The present invention further provides a method for identifying a drug that enhances or diminishes the ability of STAT protein dimers to induce the expression of a gene operably under the control of a promoter containing at least two adjacent weak binding sites for STAT protein dimers. One such embodiment comprises selecting a potential drug by performing rational drug design with the three-dimensional structure determined for a crystal of the present invention. The selecting is preferably performed in conjunction with computer modeling. The level of expression of a first reporter gene and a second reporter gene contained by a host cell in the presence and absence of the potential drug is determined. The first reporter gene is operably linked to a first promoter containing at least two adjacent weak binding sites for STAT protein dimers, and the second reporter gene is operably linked to a second promoter comprising at least one strong binding site for a STAT protein dimer. The binding of STAT protein dimers to the two adjacent weak binding sites induces the expression of the first reporter gene, and the binding of the STAT protein dimer to the strong binding site induces the expression of the second reporter gene. In addition the host cell either naturally contains STAT protein dimers or is modified and/or induced to contain them. The level of expression of the first reporter gene is then compared with that of the second reporter gene in the presence and absence of the potential drug. When the presence of the potential drug results in an increase in the level of expression of the first reporter gene but not that of the second reporter gene, the potential drug is identified as a drug that enhances the ability of STAT protein dimers to induce the expression of a gene operably under the control of a promoter containing at least two adjacent weak binding sites for STAT protein dimers. On the other hand, when the presence of a potential drug results in a decrease in the level of expression of the first reporter gene but not that of the second reporter gene, the potential drug is identified as a drug that inhibits the ability of STAT protein dimers to induce the expression of a gene operably under the control of a promoter containing at least two adjacent weak binding sites for STAT protein dimers.

In a preferred embodiment of this type, the method further comprises growing a supplemental crystal containing a protein-drug complex formed between the dimeric N-terminal domain and the drug. The crystal effectively diffracts X-rays allowing the determination of the atomic coordinates of the protein-ligand complex to a resolution of greater than 5.0 Angstroms, preferably to a resolution of greater than 3.0 Angstroms and more preferably to a resolution of greater than 2.0 Angstroms. The three-dimensional structure of the supplemental crystal is then determined with molecular replacement analysis.

A drug is selected by performing rational drug design with the three-dimensional structure determined for the supplemental crystal. The selecting is preferably performed in conjunction with computer modeling.

In an alternative embodiment, the first reporter gene is contained by a first host cell, and the second reporter gene is contained by a second host cell. In this case, both the first host cell and second host cell contain STAT protein dimers. In one embodiment, the weak STAT binding sites are from sites present in the regulatory regions of the MIG gene. In another embodiment the weak STAT binding sites are from sites present in the regulatory regions of the c-fos gene. In still another embodiment the weak STAT binding sites are from sites present in the regulatory regions of the interferon-γ gene. In a related embodiment the mutated cfos-promotor element, the M67 site [Wagner et al., EMBO J. 9:4477 (1990)] is used as the strong STAT binding site. In another embodiment of this type the strong STAT binding site is the S1 site [Horvath et al., Genes & Devel. 9:984 (1995)]. In still another such embodiment strong STAT binding site is obtained from the IRF-1 gene promoter. In preferred embodiments, the host cell or host cells are mammalian cells.

These and other aspects of the present invention will be better appreciated by reference to the following drawings and Detailed Description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a schematic representation of two STAT dimers bound to adjacent target sites. Interactions between N-domains (N, circled) allow the dimers to bind to each other. Phosphotyrosines are indicated as “Y” with encircled P symbols. DBD: DNA binding domain; SH2: SH2 domain; TAD: transactivation domain.

FIG. 1B shows the sequence alignment of the conserved N-terminal domain of the STAT family and secondary structure of the N-terminal domain of STAT-4. Human (hSTAT), murine (mSTAT), and Drosophila (DSTAT) proteins are included. The numbering is according to STAT-4, and the following sequences are shown: mSTAT4 (SEQ ID NO:9), mSTAT1 (SEQ ID NO:7), hSTAT2 (SEQ ID NO:2), mSTAT3 (SEQ ID NO:8), mSTAT5a (SEQ ID NO:10), mSTAT5b (SEQ ID NO:11), mSTAT6 (SEQ ID NO:12), dSTAT (SEQ ID NO:13) α-Helices α1 to α8 are drawn as cylinders. The blackened part of helix α2 indicates a 3₁₀ helix. Invariant residues are highlighted with an asterisk below the alignment. Conserved residues in the hydrophobic core are marked with filled circles above the STAT-4 sequence. The following amino acid exchanges are considered conserved: Q/N, D/E, I/L/V, R/K/H, Y/F, S/T. Residues in helices α6 and α7 that contribute to the packing of the coiled-coil are boxed and their position in the helical repeats is indicated (a or d).

FIG. 2: Tertiary structure of the amino-terminal domain of STAT-4. FIG. 2A depicts the overall representation of two monomers (green and gray) in the crystallographic dimer, viewed approximately orthogonal to the molecular 2-fold axis, which is vertical. The ring-shaped N-terminal element is colored red in one monomer. FIG. 2B depicts the orthogonal view of one of the N-terminal domains shown in FIG. 2A, showing details of the architecture of the ring-shaped element. Side chains that participate in a charge stabilized hydrogen-bond network are shown in a ball-and-stick representation. The side chain and backbone carbonyl of buried Arg 31 are shown in magenta. For clarity, the indole ring of the invariant residue Trp 4 that seals off this arrangement on the proximal side is drawn with thinner bonds. The blue sphere denotes a buried water molecule. Hydrogen-bonds are indicated by dotted lines. Oxygen, nitrogen, and carbon atoms are colored red, blue and yellow, respectively. Q3-N marks the position of the backbone amide group of residue Gln 3. The light red colored segment of helix α2 highlights its 3₁₀ helical conformation. FIGS. 2, 3B, and 3C were created with the program RIBBONS v2.0 [Carson, J. Appl. Cryst. 24:958 (1991)].

FIG. 3A depicts the surface representation of the N-terminal domain indicating the wedge shaped groove and the dimerization interface. Shown are two monomers of a dimer with the left one rotated 90° around the vertical axis away from the original position in the dimer. Note the hook-like appearance of the monomer with the coiled-coil of helices α6 and α7 pointing out of the planar surface formed by the ring-shaped element comprising the amino-terminal 40 residues. Residues from three separate regions of the N-terminal domain make direct or water mediated contacts in the dimer and are color-coded according to their position. Interface residues at the amino-terminus are in green, those in helices α3 and α4 are in blue and amino acids located in helix α6 are yellow. The position of the critical Trp 37 is highlighted in red. The Figure was created using GRASP [Nicholls et al., Proteins: Struct. Funct. and Genetics 11:281 (1991)].

FIG. 3B shows a view at the dimerization interface with amino acids represented as ball-and-stick models and the backbone as ribbon. The monomer is in the same orientation as the one on the right of panel of FIG. 3A. Side chains are colored according to their position following the same convention as in FIG. 3A; backbone ribbon is colored as in FIG. 2B with the first 40 residues highlighted in red. Note the entirely polar nature of the interface. Leu 33 makes a backbone carbonyl group contact and its position is represented by the filled circle. In the STAT-4 recombinant N-terminal domain used for crystallization, Met 1 was replaced with Gly plus four additional small amino acids, one of which (Gly-1) is visible in the electron density map (see Methods in the EXAMPLE below). In the crystals the amino terminus of Gly-1 is part of the dimer interface, possibly substituting for the native Met 1.

FIG. 3C shows the close-up stereo view of intermolecular hydrogen bonding network in the dimer. Selected side chains surrounding the conserved Trp 37 (magenta) in helices α4 and α6 of two monomers (green and grey) are shown. Trp 37 makes direct (E 66′) and water mediated contacts (Q 63′). Water molecules are depicted as blue spheres.

FIG. 4 indicates the importance of the invariant residue Trp 37 for STAT-1 tetramerization and mediation of gene activation.

FIG. 4A depicts the gel mobility shift that shows that a single point mutation disrupts STAT-1 cooperative DNA-binding. Comparison of tetramer stability between wild type (WT) STAT-1 (lanes 3 and 4) and the W37A mutant (lanes 1 and 2). Radiolabeled DNA containing a tandem binding site was preincubated with equal amounts of active protein of either tyrosine-phosphorylated WT-STAT or the mutant protein followed by a chase with excess (30 fold) unlabeled oligonucleotide for the indicated amount of time. The wild type STAT-1 shows the expected formation and stabilization of 2× (dimeric) complexes relative to the dimeric protein/DNA complex. The W37A mutant does form dimeric complexes, but only very little of the slower migrating 2× (dimeric) species. No increased stability of the 2× (dimeric) complex relative to the dimer is observed. Samples loaded at the later time point (15 minutes) were electrophoresed for shorter times and therefore run higher on the gel. The position of unbound oligonucleotide is marked (free).

FIG. 4B show the effect of STAT-1 W37A mutation on interferon-γ (IFN-γ) stimulated gene activation in vivo. U3A cells that are lacking STAT-1 [Miller et al., EMBO J. 12:4221 (1993)] were transfected with expression clones containing either wild type or mutant STAT-1 (see Methods of the EXAMPLE, below) along with a luciferase reporter containing a tandem STAT binding site as an enhancer. After stimulation with IFN-γ for 10 hours, luciferase expression was determined spectroscopically (represented as bars). Cells transfected with the wild type protein show an about 2-fold increase in luciferase expression. In contrast, the tetramerization deficient W37A mutant does not markedly increase transcription over background levels from an enhancer with a tandem binding site. Each bar represents ten individual parallel experiments. Error bars denote standard deviation from the mean.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides the first three-dimensional structural information regarding the important family of transcription factors known as STATS. More particularly, the present invention provides a crystalline form of the N-terminal cooperative domain (more simply denoted as the N-terminal domain) of a STAT protein of sufficient quality to perform meaningful X-ray crystallographic measurements. In addition, the present invention provides a method of preparing such crystals.

In addition the present invention provides a method of using the crystals and the crystallographic measurements for drug discovery and development. These methods include procedures for screening drugs that either enhance or inhibit STAT-STAT dimer interactions, which can have a critical effect on the transcription of the specific genes under the control of STAT proteins. For example, antagonists of the STAT N-terminal dimer interaction antagonize STAT functions dependent on this aspect of STAT behavior. Drugs that are antagonists would be useful for the treatment of a variety of disease states, including but not limited to, inflammation, allergy, asthma, and leukemias. On the other hand, drugs that are found to be agonists would stabilize the N-terminal interaction between STAT dimers, thereby enhancing this aspect of the STAT function. Such drugs may therefore have utility in the treatment of anemias, neutropenias, thrombocytopenia, cancer, obesity, viral diseases and growth retardation, or other diseases characterized by a insufficient STAT activity.

Therefore, if appearing herein, the following terms shall have the definitions set out below.

As used herein the “α4-tryptophan” is the conserved tryptophan common in all STAT proteins found in the N-terminal domain in an alpha helix defined as α4 in FIG. 1. For mStat4 the α4-tryptophan is W₃₇, of the amino acid sequence. Similarly, the α4-glutamic acid is the conserved glutamic acid common in all STAT proteins found in the N-terminal domain in an alpha helix defined as α4 in FIG. 1. For mStat4 the α4 glutamic acid is E₃₉ of the amino acid sequence.

As used herein a the term “STAT protein” includes a particular family of transcription factor consisting of the Signal Transducers and Activators of Transcription proteins. These proteins have been defined in International Patent Publication Nos. WO 93/19179 (Sep. 30, 1993, by James E. Darnell, Jr. et al.), WO 95/08629 (Mar. 30, 1995, by James E. Darnell, Jr. et al.) and United States application having a Ser. No. 08/212,184, filed on Mar. 11, 1994, entitled, “Interferon Associated Receptor Recognition Factors, Nucleic Acids Encoding the Same and Methods of Use Thereof” by James E. Darnell, Jr. et al., all of which are incorporated by reference in their entireties, herein. Currently, there are seven STAT family members which have been identified, numbered STAT 1, 2, 3, 4, 5A, 5B, and 6. STAT proteins include proteins derived from alternative splice sites such as Human STAT1α and STAT1β, i.e., STAT1β is a shorter protein than STAT1α and is translated from an alternatively spliced mRNA. Modified STAT proteins and functional fragments of STAT proteins are included in the present invention.

As used herein the terms “phosphorylated” and “nonphosphorylated” as used in conjunction with or in reference to a STAT protein denote the phosphorylation state of a particular tyrosine residue of the STAT proteins (e.g., Tyr 701 of STAT1). When STAT proteins are phosphorylated, they form homo- or heterodimeric structures in which the phosphotyrosine of one partner binds to the SRC homology domain (SH2) of the other. In their natural environment the newly formed dimer then translocates from the cytoplasm to the nucleus, binds to a palindromic GAS sequence, thereby activating transcription

The “N-terminal domain” of a STAT protein is used interchangeably herein with the “N-terminal cooperative domain” and refers to the N-terminal portion of a STAT protein involved in STAT protein dimer-dimer interaction at a weak STAT DNA binding site. Preferably the amino acid of the N-terminal domain comprises SEQ ID NO:1. In one particular embodiment the STAT protein is STAT-4 comprising amino acids 2-123 of SEQ ID NO:9.

General Techniques for Constructing Nucleic Acids That Express Recombinant STAT Proteins

In accordance with the present invention there may be employed conventional molecular biology, microbiology, and recombinant DNA techniques within the skill of the art. Such techniques are explained fully in the literature. See, e.g., Sambrook, Fritsch & Maniatis, Molecular Cloning: A Laboratory Manual, Second Edition (1989) Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N. Y. (herein “Sambrook et al., 1989”); DNA Cloning: A Practical Approach, Volumes I and II (D. N. Glover ed. 1985); Oligonucleotide Synthesis (M. J. Gait ed. 1984); Nucleic Acid Hybridization [B. D. Hames & S. J. Higgins eds. (1985)]; Transcription And Translation [B. D. Hames & S. J. Higgins, eds. (1984)]; Animal Cell Culture [R. I. Freshney, ed. (1986)]; Immobilized Cells And Enzymes [IRL Press, (1986)]; B. Perbal, A Practical Guide To Molecular Cloning (1984); F. M. Ausubel et al. (eds.), Current Protocols in Molecular Biology, John Wiley & Sons, Inc. (1994).

Therefore, if appearing herein, the following terms shall have the definitions set out below.

As used herein, the term “gene” refers to an assembly of nucleotides that encode a polypeptide, and includes cDNA and genomic DNA nucleic acids.

A “vector” is a replicon, such as plasmid, phage or cosmid, to which another DNA segment may be attached so as to bring about the replication of the attached segment. A “replicon” is any genetic element (e.g., plasmid, chromosome, virus) that functions as an autonomous unit of DNA replication in vivo, i.e., capable of replication under its own control.

A “cassette” refers to a segment of DNA that can be inserted into a vector at specific restriction sites. The segment of DNA encodes a polypeptide of interest, and the cassette and restriction sites are designed to ensure insertion of the cassette in the proper reading frame for transcription and translation.

A cell has been “transfected” by exogenous or heterologous DNA when such DNA has been introduced inside the cell.

A “nucleic acid molecule” refers to the phosphate ester polymeric form of ribonucleosides (adenosine, guanosine, uridine or cytidine; “RNA molecules”)or deoxyribonucleosides (deoxyadenosine, deoxyguanosine, deoxythymidine, or deoxycytidine; “DNA molecules”), or any phosphoester analogues thereof, such as phosphorothioates and thioesters, in either single stranded form, or a double-stranded helix. Double stranded DNA-DNA, DNA-RNA and RNA-RNA helices are possible. The term nucleic acid molecule, and in particular DNA or RNA molecule, refers only to the primary and secondary structure of the molecule, and does not limit it to any particular tertiary forms. Thus, this term includes double-stranded DNA found, inter alia, in linear or circular DNA molecules (e.g., restriction fragments), plasmids, and chromosomes. In discussing the structure of particular double-stranded DNA molecules, sequences may be described herein according to the normal convention of giving only the sequence in the 5′ to 3′ direction along the nontranscribed strand of DNA (ie., the strand having a sequence homologous to the mRNA). A “recombinant DNA molecule” is a DNA molecule that has undergone a molecular biological manipulation.

A nucleic acid molecule is “hybridizable” to another nucleic acid molecule, such as a cDNA, genomic DNA, or RNA, when a single stranded form of the nucleic acid molecule can anneal to the other nucleic acid molecule under the appropriate conditions of temperature and solution ionic strength (see Sambrook et al., supra). The conditions of temperature and ionic strength determine the “stringency” of the hybridization. For preliminary screening for homologous nucleic acids, low stringency hybridization conditions, corresponding to a T_(m) of 55°, can be used, e.g., 5×SSC, 0.1% SDS, 0.25% milk, and no formamide; or 30% formnamide, 5×SSC, 0.5% SDS). Moderate stringency hybridization conditions correspond to a higher T_(m), e.g., 40% formamide, with 5× or 6×SCC. High stringency hybridization conditions correspond to the highest T_(m), e.g., 50% formamide, 5× or 6×SCC. Hybridization requires that the two nucleic acids contain complementary sequences, although depending on the stringency of the hybridization, mismatches between bases are possible. The appropriate stringency for hybridizing nucleic acids depends on the length of the nucleic acids and the degree of complementation, variables well known in the art. The greater the degree of similarity or homology between two nucleotide sequences, the greater the value of T_(m) for hybrids of nucleic acids having those sequences. The relative stability (corresponding to higher T_(m)) of nucleic acid hybridizations decreases in the following order: RNA:RNA, DNA:RNA, DNA:DNA. For hybrids of greater than 100 nucleotides in length, equations for calculating T_(m) have been derived (see Sambrook et al., supra, 9.50-0.51). For hybridization with shorter nucleic acids, i.e., oligonucleotides, the position of mismatches becomes more important, and the length of the oligonucleotide determines its specificity (see Sambrook et al., supra, 11.7-11.8). Preferably a minimum length for a hybridizable nucleic acid is at least about 12 nucleotides; preferably at least about 18 nucleotides; and more preferably the length is at least about 27 nucleotides; and most preferably 36 nucleotides.

In a specific embodiment, the term “standard hybridization conditions” refers to a T_(m) of 55° C., and utilizes conditions as set forth above. In a preferred embodiment, the T_(m) is 60° C.; in a more preferred embodiment, the T_(m) is 65° C.

A DNA “coding sequence” is a double-stranded DNA sequence which is transcribed and translated into a polypeptide in a cell in vitro or in vivo when placed under the control of appropriate regulatory sequences. The boundaries of the coding sequence are determined by a start codon at the 5′ (amino) terminus and a translation stop codon at the 3′ (carboxyl) terminus. A coding sequence can include, but is not limited to, prokaryotic sequences and synthetic DNA sequences. If the coding sequence is intended for expression in a eukaryotic cell, a polyadenylation signal and transcription termination sequence will usually be located 3′ to the coding sequence.

Transcriptional and translational control sequences are DNA regulatory sequences, such as promoters, enhancers, terminators, and the like, that provide for the expression of a coding sequence in a host cell. In eukaryotic cells, polyadenylation signals are control sequences.

A “promoter sequence” is a DNA regulatory region capable of binding RNA polymerase in a cell and initiating transcription of a downstream (3′ direction) coding sequence. For purposes of defining the present invention, the promoter sequence is bounded at its 3′ terminus by the transcription initiation site and extends upstream (5′ direction) to include the minimum number of bases or elements necessary to initiate transcription at levels detectable above background. Within the promoter sequence will be found a transcription initiation site (conveniently defined for example, by mapping with nuclease S1), as well as protein binding domains (consensus sequences) responsible for the binding of RNA polymerase.

A coding sequence is “under the control” of transcriptional and translational control sequences in a cell when RNA polymerase transcribes the coding sequence into mRNA, which is then trans-RNA spliced and translated into the protein encoded by the coding sequence.

As used herein, the term “homologous” in all its grammatical forms refers to the relationship between proteins that possess a “common evolutionary origin,” including proteins from superfamilies (e.g., the immunoglobulin superfamily) and homologous proteins from different species (e.g., myosin light chain, etc.) (Reeck et al., 1987, Cell 50:667). Such proteins have sequence homology as reflected by their high degree of sequence similarity.

Accordingly, the term “sequence similarity” in all its grammatical forms refers to the degree of identity or correspondence between nucleic acid or amino acid sequences of proteins that may or may not share a common evolutionary origin (see Reeck et al., supra). However, in common usage and in the instant application, the term “homologous,” when modified with an adverb such as “highly,” may refer to sequence similarity and not a common evolutionary origin.

The term “corresponding to” is used herein to refer similar or homologous sequences, whether the exact position is identical or different from the molecule to which the similarity or homology is measured. Thus, the term “corresponding to” refers to the sequence similarity, and not the numbering of the amino acid residues or nucleotide bases.

A gene encoding a STAT protein, whether genomic DNA or cDNA, can be isolated from any animal source, particularly from a mammal. Methods for obtaining the STAT protein gene are well known in the art, as described above (see, e.g., Sambrook et al., 1989, supra).

A “heterologous nucleotide sequence” as used herein is a nucleotide sequence that is added to a nucleotide sequence of the present invention by recombinant methods to form a nucleic acid which is not naturally formed in nature. Such nucleic acids can encode chimeric and/or fusion proteins. Thus the heterologous nucleotide sequence can encode peptides and/or proteins which contain regulatory and/or structural properties. In another such embodiment the heterologous nucleotide can encode a protein or peptide that functions as a means of detecting the protein or peptide encoded by the nucleotide sequence of the present invention after the recombinant nucleic acid is expressed. In still another such embodiment the heterologous nucleotide can function as a means of detecting a nucleotide sequence of the present invention. A heterologous nucleotide sequence can comprise non-coding sequences including restriction sites, regulatory sites, promoters and the like.

The present invention also relates to cloning vectors containing genes encoding analogs and derivatives of the STAT protein, including modified STAT proteins of the invention, that have the same or homologous functional activity as STAT protein, and homologs thereof.

The production and use of derivatives and analogs related to the STAT protein are within the scope of the present invention.

STAT protein derivatives and analogs as described above can be made by altering encoding nucleic acid sequences by substitutions, e.g. replacing the α4-tryptophan or α4-glutamic acid with an alanine, or additions or deletions that provide for functionally equivalent or specifically modified molecules.

Due to the degeneracy of nucleotide coding sequences, other DNA sequences which encode substantially the same amino acid sequence as a nucleic acid encoding a modified STAT protein or an N-terminal STAT protein fragment of the present invention (including the fragment which lacks the α4-tryptophan) may be used in the practice of the present invention. These include but are not limited to allelic genes, homologous genes from other species, which are altered by the substitution of different codons that encode the same amino acid residue within the sequence, thus producing a silent change. Likewise, the modified STAT protein derivatives of the invention include, but are not limited to, those containing, as a primary amino acid sequence, all or part of the amino acid sequence of a STAT protein including altered sequences in which functionally equivalent amino acid residues are substituted for residues within the sequence resulting in a conservative amino acid substitution. For example, one or more amino acid residues within the sequence can be substituted by another amino acid of a similar polarity, which acts as a functional equivalent, resulting in a silent alteration. Substitutes for an amino acid within the sequence may be selected from other members of the class to which the amino acid belongs. For example, the nonpolar (hydrophobic) amino acids include alanine, leucine, isoleucine, valine, proline, phenylalanine, tryptophan and methionine. Amino acids containing aromatic ring structures are phenylalanine, tryptophan, and tyrosine. The polar neutral amino acids include glycine, serine, threonine, cysteine, tyrosine, asparagine, and glutamine. The positively charged (basic) amino acids include arginine, lysine and histidine. The negatively charged (acidic) amino acids include aspartic acid and glutamic acid.

Particularly preferred conserved amino acid exchanges are:

(a) Lys for His or for Arg or vice versa such that a positive charge may be maintained;

(b) Glu for Asp or vice versa such that a negative charge may be maintained;

(c) Ser for Thr or vice versa such that a free —OH can be maintained;

(d) Gln for Asn or vice versa such that a free NH₂ can be maintained;

(e) Ile for Leu or for Val or vice versa as roughly equivalent hydrophobic amino acids; and

(f) Phe for Tyr or vice versa as roughly equivalent aromatic amino acids.

Non-conserved amino acid substitutions may also be introduced to substitute an amino acid with a particularly preferable property. For example, a Cys may be introduced to provide a potential site for disulfide bridges with another Cys. A His may be introduced as a particular “catalytic” site (i.e., His can act as an acid or base and is the most common amino acid in biochemical catalysis). Pro may be introduced because of its particularly planar structure, which induces β-turns in the protein's structure.

The genes encoding STAT proteins, and derivatives and analogs thereof can be produced by various methods known in the art. The manipulations which result in their production can occur at the gene or protein level. For example, the cloned N-terminal domain of a STAT protein gene sequence can be modified by any of numerous strategies known in the art (Sambrook et al., 1989, supra). The sequence can be cleaved at appropriate sites with restriction endonuclease(s), followed by further enzymatic modification if desired, isolated, and ligated in vitro. In the production of the gene encoding a derivative or analog of a STAT protein care should be taken to ensure that the modified gene remains within the same translational reading frame as the STAT protein gene, uninterrupted by translational stop signals, in the gene region where the desired activity is encoded.

Additionally, the STAT protein-encoding nucleic acid sequence can be mutated in vitro or in vivo, to create and/or destroy translation, initiation, and/or termination sequences, or to create variations in coding regions and/or form new restriction endonuclease sites or destroy preexisting ones, to facilitate further in vitro modification. Any technique for mutagenesis known in the art can be used, including but not limited to, in vitro site-directed mutagenesis (Hutchinson, C., et al., 1978, J. Biol. Chem. 253:6551; Zoller and Smith, 1984, DNA 3:479-488; Oliphant et al., 1986, Gene 44:177; Hutchinson et al., 1986, Proc. Natl. Acad. Sci. U.S.A. 83:710), use of TAB® linkers (Pharmacia), etc. PCR techniques are preferred for site directed mutageriesis (see Higuchi, 1989, “Using PCR to Engineer DNA”, in PCR Technology: Principles and Applications for DNA Amplification, H. Erlich, ed., Stockton Press, Chapter 6, pp. 61-70).

The identified and isolated gene can then be inserted into an appropriate cloning vector. A large number of vector-host systems known in the art may be used. Possible vectors include, but are not limited to, plasmids or modified viruses, but the vector system must be compatible with the host cell used. Examples of vectors include, but are not limited to, E. coli, bacteriophages such as lambda derivatives, or plasmids such as pBR322 derivatives or pUC plasmid derivatives, e.g., pGEX vectors, pmal-c, pFLAG, etc. The insertion into a cloning vector can, for example, be accomplished by ligating the DNA fragment into a cloning vector which has complementary cohesive termini. However, if the complementary restriction sites used to fragment the DNA are not present in the cloning vector, the ends of the DNA molecules may be enzymatically modified. Alternatively, any site desired may be produced by ligating nucleotide sequences (linkers) onto the DNA termini; these ligated linkers may comprise specific chemically synthesized oligonucleotides encoding restriction endonuclease recognition sequences. Recombinant molecules can be introduced into host cells via transformation, transfection, infection, electroporation, etc., so that many copies of the gene sequence are generated. Preferably, the cloned gene is contained on a shuttle vector plasmid, which provides for expansion in a cloning cell, e.g. E. coli, and facile purification for subsequent insertion into an appropriate expression cell line, if such is desired. For example, a shuttle vector, which is a vector that can replicate in more than one type of organism, can be prepared for replication in both E. coli and Saccharomyces cerevisiae by linking sequences from an E. coli plasmid with sequences form the yeast 2μ plasmid.

In an alternative method, the desired gene may be identified and isolated after insertion into a suitable cloning vector in a “shot gun” approach. Enrichment for the desired gene, for example, by size fractionation, can be done before insertion into the cloning vector.

Expression of STAT Proteins

The nucleotide sequence coding for a STAT protein, or functional fragment, including the N-terminal peptide fragment of a STAT protein, derivatives or analogs thereof, including a chimeric protein, thereof, can be inserted into an appropriate expression vector, i.e., a vector which contains the necessary elements for the transcription and translation of the inserted protein-coding sequence. Such elements are termed herein a “promoter.” Thus, the nucleic acid encoding a STAT protein of the invention or functional fragment, including the N-terminal peptide fragment of a STAT protein, derivatives or analogs thereof, is operationally associated with a promoter in an expression vector of the invention. Both cDNA and genomic sequences can be cloned and expressed under control of such regulatory sequences. An expression vector also preferably includes a replication origin. The necessary transcriptional and translational signals can be provided on a recombinant expression vector. As detailed below, all genetic manipulations described for the STAT gone in this section, may also be employed for genes encoding a functional fragment, including the N-terminal domain peptide fragment of a STAT protein, derivatives or analogs thereof, including a chimeric protein, thereof.

Potential host-vector systems include but are not limited to mammalian cell systems infected with virus (e.g., vaccinia virus, adenovirus, etc.); insect cell systems infected with virus (e.g., baculovirus); microorganisms such as yeast containing yeast vectors; or bacteria transformed with bacteriophage, DNA, plasmid DNA, or cosmid DNA. The expression elements of vectors vary in their strengths and specificities. Depending on the host-vector system utilized, any one of a number of suitable transcription and translation elements may be used.

A recombinant STAT protein of the invention, may be expressed chromosomally, after integration of the coding sequence by recombination. In this regard, any of a number of amplification systems may be used to achieve high levels of stable gene expression (See Sambrook et al., 1989, supra).

The cell into which the recombinant vector comprising the nucleic acid encoding STAT protein is cultured in an appropriate cell culture medium under conditions that provide for expression of STAT protein by the cell.

Any of the methods previously described for the insertion of DNA fragments into a cloning vector may be used to construct expression vectors containing a gene consisting of appropriate transcriptional/translational control signals and the protein coding sequences. These methods may include in vitro recombinant DNA and synthetic techniques and in vivo recombination (genetic recombination).

Expression of STAT protein may be controlled by any promoter/enhancer element known in the art, but these regulatory elements must be functional in the host selected for expression.

Expression vectors containing a nucleic acid encoding a STAT protein of the invention can be identified by four general approaches: (a) PCR amplification of the desired plasmid DNA or specific mRNA, (b) nucleic acid hybridization, (c) presence or absence of selection marker gene functions, and (d) expression of inserted sequences. In the first approach, the nucleic acids can be amplified by PCR to provide for detection of the amplified product. In the second approach, the presence of a foreign gene inserted in an expression vector can be detected by nucleic acid hybridization using probes comprising sequences that are homologous to an inserted marker gene. In the third approach, the recombinant vector/host system can be identified and selected based upon the presence or absence of certain “selection marker” gene functions (e.g., β-galactosidase activity, thymidine kinase activity, resistance to antibiotics, transformation phenotype, occlusion body formation in baculovirus, etc.) caused by the insertion of foreign genes in the vector. In another example, if the nucleic acid encoding STAT protein is inserted within the “selection marker” gene sequence of the vector, recombinants containing the STAT protein insert can be identified by the absence of the STAT protein gene function. In the fourth approach, recombinant expression vectors can be identified by assaying for the activity, biochemical, or immunological characteristics of the gene product expressed by the recombinant, provided that the expressed protein assumes a functionally active conformation.

A wide variety of host/expression vector combinations may be employed in expressing the DNA sequences of this invention. Useful expression vectors, for example, may consist of segments of chromosomal, nonchromosomal and synthetic DNA sequences. Suitable vectors include derivatives of SV40 and known bacterial plasmids, e.g., E. coli plasmids col E1, pCR1, pBR322, pMal-C2, pET, pGEX (Smith et al., 1988, Gene 67:31-40), pMB9 and their derivatives, plasmids such as RP4; phage DNAS, e.g., the numerous derivatives of phage λ, e.g., NM989, and other phage DNA, e.g., M13 and filamentous single stranded phage DNA; yeast plasmids such as the 21μ plasmid or derivatives thereof, vectors useful in eukaryotic cells, such as vectors useful in insect or mammalian cells; vectors derived from combinations of plasmids and phage DNAs, such as plasmids that have been modified to employ phage DNA or other expression control sequences; and the like.

For example, in a baculovirus expression systems, both non-fusion transfer vectors, such as but not limited to pVL941 (BamH1 cloning site; Summers), pVL1393 (BamH1, SmaI, XbaI, EcoR1, NotI, XmaIII, BglII, and PstI cloning site; Invitrogen), pVL1392 (BglII, PstI, NotI, XmaIII, EcoRI, XbaI, SmaI, and BamH1 cloning site; Summers and Invitrogen), and pBlueBacIII (BamH1, BglII, PstI, NcoI, and HindIII cloning site, with blue/white recombinant screening possible; Invitrogen), and fusion transfer vectors, such as but not limited to pAc700 (BamH1 and KpnI cloning site, in which the BamH1 recognition site begins with the initiation codon; Summers), pAc701 and pAc702 (same as pAc700 , with different reading frames), pAc360 (BamH1 cloning site 36 base pairs downstream of a polyhedron initiation codon; Invitrogen(195)), and pBlueBacHisA, B, C (three different reading frames, with BamH1, BglII, PstI, NcoI, and HindIII cloning site, an N-terminal peptide for ProBond purification, and blue/white recombinant screening of plaques; Invitrogen (220)) can be used.

Mammalian expression vectors contemplated for use in the invention include vectors with inducible promoters, such as the dihydrofolate reductase (DHFR) promoter, e.g., any expression vector with a DHFR expression vector, or a DHFR/methotrexate co-amplification vector, such as pED (PstI, SalI, SbaI, SmaI, and EcoRI cloning site, with the vector expressing both the cloned gene and DHFR; see Kaufman, Current Protocols in Molecular Biology, 16.12 (1991). Alternatively, a glutamine synthetase/methionine sulfoximine co-amplification vector, such as pEE14 (HindIII, XbaI, SmaI, SbaI, EcoRI, and BclI cloning site, in which the vector expresses glutamine synthase and the cloned gene; Celltech). In another embodiment, a vector that directs episomal expression under control of Epstein Barr Virus (EBV) can be used, such as pREP4 (BamH1, SfiI, XhoI, NotI, NheI, HindIII, NheI, PvuII, and KpnI cloning site, constitutive RSV-LTR promoter, hygromycin selectable marker; Invitrogen), pCEP4 (BamH1, SfiI, Xhol, NotI, NheI, HindIII, NheI, PvuII, and KpnI cloning site, constitutive hCMV immediate early gene, hygromycin selectable marker; Invitrogen), pMEP4 (KpnI, PvuI, NheI, HindIII, NotI, XhoI, SfiI, BamH1 cloning site, inducible methallothionein IIa gene promoter, hygromycin selectable marker: Invitrogen), pREP8 (BamH1, XhoI, NotI, HindIII, NheI, and KpnI cloning site, RSV-LTR promoter, histidinol selectable marker; Invitrogen), pREP9 (KpnI, NheI, HindIII, NotI, XhoI, SfiI, and BamHI cloning site, RSV-LTR promoter, G418 selectable marker; Invitrogen), and pEBVHis (RSV-LTR promoter, hygromycin selectable marker, N-terminal peptide purifiable via ProBond resin and cleaved by-enterokinase; Invitrogen). Selectable mammalian expression vectors for use in the invention include pRc/CMV (HindII, BstXI, NotI, SbaI, and ApaI cloning site, G418 selection; Invitrogen), pRc/RSV (HindIII, SpeI, BstXI, NotI, XbaI cloning site, G418 selection; Invitrogen), and others. Vaccinia virus mammalian expression vectors (see, Kaufman, 1991, supra) for use according to the invention include but are not limited to pSC11 (SmaI cloning site, TK- and ⊖-gal selection), pMJ601 (SalI, SmaI, AflI, NarI, BspMII, BamHI, ApaI, NheI, SacII, KpnI, and HindIII cloning site; TK- and β-gal selection), and pTKgptF1S (EcoRI, PstI, SalI, AccI, HindII, SbaI, BamHI, and Hpa cloning site, TK or XPRT selection).

Yeast expression systems can also be used according to the invention to express OB polypeptide. For example, the non-fusion pYES2 vector (XbaI, SphI, ShoI, NotI, GstXI, EcoRI, BstXI, BamH1, Sacl, Kpn1, and HindIII cloning sit; Invitrogen) or the fusion pYESHisA, B, C (XbaI, SphI, ShoI, NotI, BstXI, EcoRI, BamH1, SacI, KpnI, and HindIII cloning site, N-terminal peptide purified with ProBond resin and cleaved with enterolinase; Invitrogen), to mention just two, can be employed according to the present invention.

Once a particular recombinant DNA molecule is identified and isolated, several methods known in the art may be used to propagate it. Once a suitable host system and growth conditions are established, recombinant expression vectors can be propagated and prepared in quantity. As previously explained, the expression vectors which can be used include, but are not limited to, the following vectors or their derivatives: human or animal viruses such as vaccinia virus or adenovirus; insect viruses such as baculovirus; yeast vectors; bacteriophage vectors (e.g., lambda), and plasmid and cosmid DNA vectors, to name but a few.

Vectors are introduced into the desired host cells by methods known in the art, e.g., transfection, electroporation, microinjection, transduction, cell fusion, DEAE dextran, calcium phosphate precipitation, lipofection (lysosome fusion), use of a gene gun, or a DNA vector transporter (see, e.g., Wu et al., 1992, J. Biol. Chem. 267:963-967; Wu and Wu, 1988, J. Biol. Chem. 263:14621-14624; Hartmut et al., Canadian Patent Application No. 2,012,31 1, filed Mar. 15, 1990).

Synthetic Polypeptides

The term “polypeptide” is used in its broadest sense to refer to a compound of two or more subunit amino acids, amino acid analogs, or peptidomimetics. The subunits are linked by peptide bonds. The STAT proteins and more particularly the N-terminal domain fragments thereof, of the present invention may be chemically synthesized.

More particularly, potential drugs that may be tested in the drug screening assays of the present invention may also be chemically synthesized. Synthetic polypeptides, prepared using the well known techniques of solid phase, liquid phase, or peptide condensation techniques, or any combination thereof, can include natural and unnatural amino acids. Amino acids used for peptide synthesis may be standard Boc (N^(α)-amino protected N^(α)-t-butyloxycarbonyl) amino acid resin with the standard deprotecting, neutralization, coupling and wash protocols of the original solid phase procedure of Merrifield (1963, J. Am. Chem. Soc. 85:2149-2154), or the base-labile N^(α)-amino protected 9-fluorenylmethoxycarbonyl (Fmoc) amino acids first described by Carpino and Han (1972, J. Org. Chem. 37:3403-3409). Both Fmoc and Boc N^(α)-amino protected amino acids can be obtained from Fluka, Bachem, Advanced Chemtech, Sigma, Cambridge Research Biochemical, Bachem, or Peninsula Labs or other chemical companies familiar to those who practice this art. In addition, the method of the invention can be used with other N^(α)-protecting groups that are familiar to those skilled in this art. Solid phase peptide synthesis may be accomplished by techniques familiar to those in the art and provided, for example, in Stewart and Young, 1984, Solid Phase Synthesis, Second Edition, Pierce Chemical Co., Rockford, Ill.; Fields and Noble, 1990, Int. J. Pept. Protein Res. 35:161-214, or using automated synthesizers, such as sold by ABS. Thus, polypeptides of the invention may comprise D-amino acids, a combination of D- and L-amino acids, and various “designer” amino acids (e.g., β-methyl amino acids, Cα-methyl amino acids, and Nα-methyl amino acids, etc.) to convey special properties. Synthetic amino acids include omithine for lysine, fluorophenylalanine for phenylalanine, and norleucine for leucine or isoleucine. Additionally, by assigning specific amino acids at specific coupling steps, α-helices, β turns, β sheets, γ-turns, and cyclic peptides can be generated.

In a further embodiment, subunits of peptides that confer useful chemical and structural properties will be chosen. For example, peptides comprising D-amino acids will be resistant to L-amino acid-specific proteases in vivo. In addition, the present invention envisions preparing peptidesthat have more well defined structural properties, and the use of peptidomimetics, and peptidomimetic bonds, such as ester bonds, to prepare peptides with novel properties. In another embodiment, a peptide may be generated that incorporates a reduced peptide bond, i.e., R₁—CH₂—NH—R₂, where R₁ and R₂ are amino acid residues or sequences. A reduced peptide bond may be introduced as a dipeptide subunit. Such a molecule would be resistant to peptide bond hydrolysis, e.g. protease activity. Such peptides would provide ligands with unique function and activity, such as extended half-lives in vivo due to resistance to metabolic breakdown, or protease activity. Furthermore, it is well known that in certain systems constrained peptides show enhanced functional activity (Hruby, 1982, Life Sciences 31:189-199; Hruby et al., 1990, Biochem J. 268:249-262); the present invention provides a method to produce a constrained peptide that incorporates random sequences at all other positions.

Constrained and Cyclic Peptides.

A constrained, cyclic or rigidized peptide may be prepared synthetically, provided that in at least two positions in the sequence of the peptide an amino acid or amino acid analog is inserted that provides a chemical functional group capable of crosslinking to constrain, cyclise or rigidize the peptide after treatment to form the crosslink. Cyclization will be favored when a turn-inducing amino acid is incorporated. Examples of amino acids capable of crosslinking a peptide are cysteine to form disulfides, aspartic acid to form a lactone or a lactam, and a chelator such as γ-carboxyl-glutamic acid (Gla) (Bachem) to chelate a transition metal and form a cross-link. Protected γ-carboxyl glutamic acid may be prepared by modifying the synthesis described by Zee-Cheng and Olson (1980, Biophys. Biochem. Res. Commun. 94:1128-1132). A peptide in which the peptide sequence comprises at least two amino acids capable of crosslinking may be treated, e.g., by oxidation of cysteine residues to form a disulfide or addition of a metal ion to form a chelate, so as to crosslink the peptide and form a constrained, cyclic or rigidized peptide.

The present invention provides strategies to systematically prepare cross-links. For example, if four cysteine residues are incorporated in the peptide sequence, different protecting groups may be used (Hiskey, 1981, in The Peptides: Analysis, Synthesis, Biology, Vol. 3, Gross and Meienhofer, eds., Academic Press: New York, pp. 137-167; Ponsanti et al., 1990, Tetrahedron 46:8255-8266). The first pair of cysteines may be deprotected and oxidized, then the second set may be deprotected and oxidized. In this way a defined set of disulfide cross-links may be formed. Alternatively, a pair of cysteines and a pair of chelating amino acid analogs maybe-incorporated so that the cross-links are of a different chemical nature.

Non-classical Amino Acids that Induce Conformnational Constraints.

The following non-classical amino acids may be incorporated in the peptide in order to introduce particular conformational motifs: 1,2,3,4-tetrahydroisoquinoline-3-carboxylate (Kazmierski et al., 1991, J. Am. Chem. Soc. 113:2275-2283); (2S,3S)-methyl-phenylalanine, (2S,3R)-methyl-phenylalanine, (2R,3S)-methyl-phenylalanine and (2R,3R)-methyl-phenylalanine (Kazmierski and Hruby, 1991, Tetrahedron Lett.); 2-aminotetrahydronaphthalene-2-carboxylic acid (Landis, 1989, Ph.D. Thesis, University of Arizona); hydroxy-1,2,3,4-tetrahydroisoquinoline-3-carboxylate (Miyake et al., 1989, J. Takeda Res. Labs. 43:53-76); β-carboline (D and L) (Kazmierslci, 1988, Ph.D. Thesis, University of Arizona); HIC (histidine isoquinoline carboxylic acid) (Zechel et al., 1991, Int. J. Pep. Protein Res. 43); and HIC (histidine cyclic urea) (Dharanipragada).

The following amino acid analogs and peptidomimetics may be incorporated into a peptide to induce or favor specific secondary structures: LL-Acp (LL-3-amino-2-propenidone-6-carboxylic acid), a β-turn inducing dipeptide analog (Kemp et al., 1985, J. Org. Chem. 50:5834-5838); β-sheet inducing analogs (Kemp et al., 1988, Tetrahedron Lett. 29:5081-5082); β-turn inducing analogs (Kemp et al., 1988, Tetrahedron Lett. 29:5057-5060); ∝-helix inducing analogs (Kemp et al., 1988, Tetrahedron Lett. 29:4935-4938); γ-turn inducing analogs (Kemp et al., 1989, J. Org. Chem. 54:109:115); and analogs provided by the following references: Nagai and Sato, 1985, Tetrahedron Lett. 26:647-650; DiMaio et al., 1989, J. Chem. Soc. Perkin Trans. p. 1687; also a Gly-Ala turn analog (Kahn et al., 1989, Tetrahedron Lett. 30:2317); amide bond isostere (Jones et al., 1988, Tetrahedron Lett. 29:3853-3856); tretrazol (Zabrocki et al., 1988, J. Am. Chem. Soc. 110:5875-5880); DTC (Samanen et al., 1990, Int. J. Protein Pep. Res. 35:501:509); and analogs taught in Olson et al., 1990, J. Am. Chem. Sci. 112:323-333 and Garvey et al., 1990, J. Org. Chem. 56:436. Conformationally restricted mimetics of beta turns and beta bulges, and peptides containing them, are described in U.S. Pat. No. 5,440,013, issued Aug. 8, 1995 to Kahn.

Crystals of the N-Terminal domain fragment of a STAT protein

Crystals of the N-terminal domain fragment of a STAT protein can be grown by a number of techniques including batch crystallization, vapor diffusion (either by sitting drop or hanging drop) and by microdialysis. Seeding of the crystals in some instances is required to obtain X-ray quality crystals. Standard micro and/or macro seeding of crystals may therefore be used.

As exemplified below hanging drops of one μl of the STAT-4 -N-terminal domain fragment (20 mg/ml, in 50 mM Hepes/HCl pH 8.0, 150 mM KCl, 2.5 mM CaCl₂, 5 mM DTT) was mixed with equal volumes of reservoir buffer containing 0.2 M Na⁺CH₃COO⁻, 0.1 M Tris/HCL pH 8.0, 17% PEG4000. Hexagonal crystals (0.2×0.2×0.2 mm) were routinely grown overnight at 20° C.

Once a crystal of the present invention is grown, X-ray diffraction data can be collected. The Example below used a MAR imaging plate detector for X-ray diffraction data collection though alternative methods may also be used. For example, crystals can be characterized by using X-rays produced in a conventional source (such as a sealed tube or a rotating anode) or using a synchrotron source. Methods of characterization include, but are not limited to, precision photography, oscillation photography and diffractometer data collection. Heavy atom derivatives such as produced with a mercurial, exemplified below, can be performed using Fuji imaging plates. Alternatively, the STAT fragment can be synthesized with selenium-methionine (Se-Met) in place of methionine, and the Se-Met multiwavelength anomalous dispersion data [Hendrickson, Science, 254:51-58 (1991)] can be collected on CHESS F2, using reverse-beam geometry to record Friedel pairs at four X-ray wavelengths, corresponding to two remote points above and below the Se absorption edge (λ₁, and λ₄) and the absorption edge inflection point (λ₂) and peak (λ₃). Selenium sites can be located using SHELXS-90 in Patterson search mode (G. M. Sheldrick). Experimental phases (α_(MAD)) can be estimated via a multiple isomorphous replacement/anomalous scattering strategy using MLPHARE (Z. Otwinowski, Southwestern University of Texas, Dallas) with three of the wavelengths treated as derivatives and one (λ₂) treated as the parent for example. In either case, data can be processed using HKL, DENZO and SCALEPACK (Z. Otwinowski and W. Minor).

In addition, X-PLOR, as used in the Example below [Brüger, X-PLOR v. 3.1 Manual, New Haven: Yale University, (1993B)] or Heavy [T. Terwilliger, Los Alamos National Laboratory] may be utilized for bulk solvent correction and B-factor scaling. After density modification and non-crystallographic averaging, the protein is built into a electron density map using the program O, as exemplified below [Jones et al., Acta Cryst., A47:110-119 (1991)]. Model building interspersed with positional and simulated annealing refinement [Brünger, 1993B, supra] can permit the an unambiguous trace and sequence assignment of the N-terminal domain fragment of the STAT protein.

Protein-Structure Based Design of Agonists and Antagonists of STAT Proteins

Once the three-dimensional structure of a crystal comprising a an N-terminal domain fragment of a STAT protein is determined, a potential ligand (antagonist or agonist) is examined through the use of computer modeling using a docking program such as GRAM, DOCK, or AUTODOCK [Dunbrack et al., 1997, supra]. This procedure can include computer fitting of potential ligands to the STAT dimer to ascertain how well the shape and the chemical structure of the potential ligand will complement or interfere with the dimer-dimer interaction. [Bugg et al., Scientific American, Dec.:92-98 (1993); West et al., TIPS, 16:67-74 (1995)]. Computer programs can also be employed to estimate the attraction, repulsion, and steric hindrance of the ligand to the dimer-dimer binding site. Generally the tighter the fit (e.g., the lower the steric hindrance, and/or the greater the attractive force) the more potent the potential drug will be since these properties are consistent with a tighter binding constant. Furthermore, the more specificity in the design of a potential drug the more likely that the drug will not interfer with other properties of the STAT protein or other proteins (particularly proteins present in the nucleus). This will minimize potential side-effects due to unwanted interactions with other proteins.

Initially a potential ligand could be obtained by screening a random peptide library produced by recombinant bacteriophage for example, [Scott and Smith, Science, 249:386-390 (1990); Cwirla et al., Proc. Natl. Acad. Sci., 87:6378-6382 (1990); Devlin et al., Science, 249:404-406 (1990)] or a chemical library. A ligand selected in this manner could be then be systematically modified by computer modeling programs until one or more promising potential ligands are identified. Such analysis has been shown to be effective in the development of HIV protease inhibitors [Lam et al., Science 263:380-384 (1994); Wlodawer et al., Ann. Rev. Biochem. 62:543-585 (1993); Appelt, Perspectives in Drug Discovery and Design 1:23-48 (1993); Erickson, Perspectives in Drug Discovery and Design 1:109-128 (1993)].

Such computer modeling allows the selection of a finite number of rational chemical modifications, as opposed to the countless number of essentially random chemical modifications that could be made, and of which any one might lead to a useful drug. Each chemical modification requires additional chemical steps, which while being reasonable for the synthesis of a finite number of compounds, quickly becomes overwhelming if all possible modifications needed to be synthesized. Thus through the use of the three-dimensional structure disclosed herein and computer modeling, a large number of these compounds can be rapidly screened on the computer monitor screen, and a few likely candidates can be determined without the laborious synthesis of untold numbers of compounds.

Once a potential ligand (agonist or antagonist) is identified it can be either selected from a library of chemicals as are commercially available from most large chemical companies including Merck, GlaxoWelcome, Bristol Meyers Squib, Monsanto/Searle, Eli Lilly, Novartis and Pharmacia UpJohn, or alternatively the potential ligand may be synthesized de novo. As mentioned above, the de novo synthesis of one or even a relatively small group of specific compounds is reasonable in the art of drug design. The prospective drug can be placed into any standard binding assay described below to test its effect on the dimeric N-terminal domain STAT-STAT interaction.

When a suitable drug is identified, a supplemental crystal can be grown which comprises a protein-ligand complex formed between an N-terminal domain of the STAT protein and the drug. Preferably the crystal effectively diffracts X-rays allowing the determination of the atomic coordinates of the protein-ligand complex to a resolution of greater than 5.0 Angstroms, more preferably greater than 3.0 Angstroms, and even more preferably greater than 2.0 Angstroms. The three-dimensional structure of the supplemental crystal can be determined by Molecular Replacement Analysis. Molecular replacement involves using a known three-dimensional structure as a search model to determine the structure of a closely related molecule or protein-ligand complex in a new crystal form. The measured X-ray diffraction properties of the new crystal are compared with the search model structure to compute the position and orientation of the protein in the new crystal. Computer programs that can be used include: X-PLOR and AMORE [J. Navaza, Acta Crystallographics ASO, 157-163 (1994)]. Once the position and orientation are known an electron density map can be calculated using the search model to provide X-ray phases. Thereafter, the electron density is inspected for structural differences and the search model is modified to conform to the new structure. Using this approach, it will be possible to use the claimed structure of the N-terminal domain STAT fragment to solve the three-dimensional structures of any such STAT fragment. Other computer programs that can be used to solve the structures of such STAT crystals include QUANTA, CHARMM; INSIGHT; SYBYL; MACROMODE; and ICM.

For all of the drug screening assays described herein further refinements to the structure of the drug will generally be necessary and can be made by the successive iterations of any and/or all of the steps provided by the particular drug screening assay.

Phage Libraries for Drug Screening.

Phage libraries have been constructed which when infected into host E. coli produce random peptide sequences of approximately 10 to 15 amino acids [Parmley and Smith, Gene 73:305-318 (1988), Scott and Smith, Science 249:386-249 (1990)]. Specifically, the phage library can be mixed in low dilutions with permissive E. coli in low melting point LB agar which is then poured on top of LB agar plates. After incubating the plates at 37° C. for a period of time, small clear plaques in a lawn of E. coli will form which represents active phage growth and lysis of the E. coli. A representative of these phages can be absorbed to nylon filters by placing dry filters onto the agar plates. The filters can be marked for orientation, removed, and placed in washing solutions to block any remaining absorbent sites. The filters can then be placed in a solution containing, for example, a radioactive N-terminal peptide fragment of a STAT protein (e.g., a fragment having an amino acid sequence comprising SEQ ID NO:1). After a specified incubation period, the filters can be thoroughly washed and developed for autoradiography. Plagues containing the phage that bind to the radioactive N-terminal peptide fragment of a STAT protein can then be identified. These phages can be further cloned and then retested for their ability to bind to the N-terminal peptide fragment of a STAT protein as before. Once the phages have been purified, the binding sequence contained within the phage can be determined by standard DNA sequencing techniques. Once the DNA sequence is known, synthetic peptides can be generated which represents these sequences.

These peptides can be tested, for example, for their ability to: (1) interfere with a dimeric STAT protein binding to a weak STAT DNA binding site; and (2) interfere with a dimeric STAT protein lacking the α4-tryptophan and/or a truncated STAT protein in which the N-terminal domain has been removed (e.g., Stat1tc, [Vinkemeier et al., EMBO J. 15: 5616 (1996)]) from binding to the same DNA binding site. If the peptide interferes in the first case but does not interfere in the latter case, it may be concluded that the peptide interferes with dimeric N-terminal STAT-STAT interaction.

The effective peptide(s) can be synthesized in large quantities for use in in vivo models and eventually in humans to modulate STAT signal transduction. It should be emphasized that synthetic peptide production is relatively non-labor intensive, easily manufactured, quality controlled and thus, large quantities of the desired product can be produced quite cheaply. Similar combinations of mass produced synthetic peptides have recently been used with great success [Patarroyo, Vaccine 10:175-178 (1990)].

Binding Assays for Drug Screening Assays

The drug screening assays of the present invention may use any of a number of assays for measuring the stability of a STAT-STAT dimeric interaction, including N-terminal dimeric STAT fragments and/or a dimeric STAT-STAT-DNA binding interaction. In one embodiment the stability of a preformed DNA-protein complex between a dimeric STAT protein and its corresponding DNA binding site is examined as follows: the formation of a complex between the STAT protein and a labeled oligonucleotide is allowed to occur and unlabelled oligonucleotides are added in vast molar excess after the reaction reaches equilibrium. At various times after the addition of unlabelled competitor DNA, aliquots are layered on a running native polyacrylamide gel to determine free and bound oligonucleotides. In one preferred embodiment the protein is STAT1α, and two different labeled DNAs are used, the natural cfos site, an example of a “weak” site, and the mutated cfos-promotor element, the M67 site [Wagner et al., EMBO J. 9:4477 (1990)] an example of a “strong” site as described below. Other examples of weak sites include those in the promoter of the MIG gene, and those in the regulatory region of the interferon-γ gene. Other examples of strong sites include those such as the selected optimum site, S1 [Horvath et al., Genes & Devel. 9:984 (1995)] or the promoter of the IRF-1 gene.

In a related binding assay, a nucleic acid containing a weak STAT binding site is placed on or coated onto a solid support. Methods for placing the nucleic acid on the solid support are well known in the art and include such things as linking biotin to the nucleic acid and linking avidin to the solid support. Dimeric STAT proteins are allowed to equilibrate with the nucleic acid andadrngs are tested to see if they disrupt or enhance the binding. Disruption leads to either a faster release of the STAT protein which may be expressed as a faster off time, and or a greater concentration of released STAT dimer. Enhancement leads to either a slower release of the STAT protein which may be expressed as a slower off time, and/or a lower concentration of released STAT protein.

The STAT protein may be labeled as described below. For example, in one embodiment radiolabeled STAT proteins are used to measure the effect of a drug on binding. In another embodiment the natural ultraviolet absorbance of the STAT protein is used. In yet another embodiment, a Biocore chip (Pharmacia) coated with the nucleic acid is used and the change in surface conductivity can be measured.

In yet another embodiment, the affect of a prospective drug (a test compound) on interactions between N-terminal domains of STATs is assayed in living cells that contain or can be induced to contain activated STAT proteins, i.e., STAT protein dimers. Cells containing a reporter gene, such as the heterologous gene for luciferase, green fluorescent protein, chloramphenicol acetyl transferase or β-galactosidase, operably linked to a promoter comprising two weak STAT binding sites are contacted with a prospective drug in the presence of a cytokine which activates the STAT(s) of interest. The amount (and/or activity) of reporter produced in the absence and presence of prospective drug is determined and compared. Prospective drugs which reduce the amount (and/or activity) of reporter produced are candidate antagonists of the N-terminal interaction, whereas prospective drugs which increase the amount (and/or activity) of reporter produced are candidate agonists. Cells containing a reporter gene operably linked to a promoter comprising strong STAT binding sites are then contacted with these candidate drugs, in the presence of a cytokine which activates the STAT(s) of interest. The amount (and/or activity) of reporter produced in the presence and absence of candidate drugs is determined and compared. Drugs which disrupt interactions between dimeric N-terminal domains of the STATs will not reduce reporter activity in this second step. Similarly, candidate drugs which enhance interactions between dimeric N-terminal domains of STATs will not increase reporter activity in this second step.

In an analogous embodiment, two reporter genes each operably under the control of one or the other of the two types promoters described above can be comprised in a single host cell as long as the expression of the two reporter gene products can be distinguished. For example, different modified forms of green fluorescent protein can be used as described in U.S. Pat. No. 5,625,048, Issued Apr. 29, 1997, hereby incorporated by reference in its entirety.

Although cells that naturally encode the STAT proteins may be used, preferably a cell is used that is transfected with a plasmid encoding the STAT protein. For example transient transfections can be performed with 50% confluent U3A cells using the calcium phosphate method as instructed by the manufacturer (Stratagene). In addition as mentioned above, the cells can also be modified to contain one or more reporter genes, a heterologous gene encoding a reporter such as luciferase, green fluorescent protein or derivative thereof, chloramphenicol acetyl transferase, β-galactosidase, etc. Such reporter genes can individually be operably linked to promoters comprising two weak STAT binding sites and/or a promoter comprising a strong STAT binding site. Assays for detecting the reporter gene products are readily available in the literature. For example, luciferase assays can be performed according to the manufacturer's protocol (Promega), and β-galactosidase assays can be performed as described by Ausubel et al., [in Current Protocols in Molecular Biology, J. Wiley & Sons, Inc. (1994)].

In one example, the transfection reaction can comprise the transfection of a cell with a plasmid modified to contain a STAT protein, such as a pcDNA3 plasmid (Invitrogen), a reporter plasmid that contains a first reporter gene, and a reporter plasmid that contains a second reporter gene. Although the preparation of such plasmids is now routine in the art, many appropriate plasmids are commercially available e.g., a plasmid with β-galactosidase is available from Stratagene.

The reporter plasmids can contain specific restriction sites in which an enhancer element having a strong STAT binding site or alternatively two tandemly arranged “weak” STAT binding sites can be inserted. In one particular embodiment, thirty-six hours after transfection of the cells with a plasmid encoding STAT-1, the cells are treated with 5 ng/ml interferon-γ Amgen for ten hours. Protein expression and tyrosine phosphorylation (to monitor STAT activation) can be determined by e.g., gel shift experiments with whole cell extracts.

Labels

Suitable labels include enzymes, fluorophores (e.g., fluorescein isothiocyanate (FITC), phycoerythrin (PE), Texas red (TR), rhodamine, free or chelated lanthanide series salts, especially Eu³⁺, to name a few fluorophores), chromophores, radioisotopes, chelating agents, dyes, colloidal gold, latex particles, ligands (e.g., biotin), and chemiluminescent agents.

When a control marker is employed, the same or different labels may be used for the test and control marker gene.

In the instance where a radioactive label, such as the isotopes ³H, ¹⁴C, ³²P, ³⁵S, ³⁶Cl, ⁵¹Cr, ⁵⁷Co, ⁵⁸Co, ⁵⁹Fe, ⁹⁰Y, ¹²⁵I, ¹³¹I, and ¹⁸⁶Re are used, known currently available counting procedures may be utilized. In the instance where the label is an enzyme, detection may be accomplished by any of the presently utilized colorimetric, spectrophotometric, fluorospectrophotometric, amperometric or gasometric techniques known in the art.

Direct labels are one example of labels which can be used according to the present invention. A direct label has been defined as an entity, which in its natural state, is readily visible, either C to the naked eye, or with the aid of an optical filter and/or applied stimulation, e.g. U.V. light to promote fluorescence. Among examples of colored labels, which can be used according to the present invention, include metallic sol particles, for example, gold sol particles such as those described by Leuvering (U.S. Pat. No. 4,313,734); dye sole particles such as described by Gribnau et al. (U.S. Pat. No. 4,373,932 and May et al. (WO 88/08534); dyed latex such as described by May, supra, Snyder (EP-A 0 280 559 and 0 281 327); or dyes encapsulated in liposomes as described by Campbell et al. (U.S. Pat. No. 4,703,017). Other direct labels include a radionucleotide, a fluorescent moiety or a luminescent moiety. In addition to these direct labeling devices, indirect labels comprising enzymes can also be used according to the present invention. Various types of enzyme linked immunoassays are well known in the art, for example, alkaline phosphatase and horseradish peroxidase, lysozyme, glucose-6-phosphate dehydrogenase, lactate dehydrogenase, urease, these and others have been discussed in detail by Eva Engvall in Enzyme Immunoassay ELISA and EMIT in Methods in Enzymology, 70:419-439 (1980) and in U.S. Pat. No. 4,857,453.

Suitable enzymes include, but are not limited to, alkaline phosphatase, β-galactosidase, green fluorescent protein and its derivatives, luciferase, and horseradish peroxidase.

Other labels for use in the invention include magnetic beads or magnetic resonance imaging labels.

The present invention may be better understood by reference to the following non-limiting Example, which is provided as exemplary of the invention. The following example is presented in order to more fully illustrate the preferred embodiments of the invention. It should in no way be construed, however, as limiting the broad scope of the invention.

EXAMPLE STRUCTURE OF THE AMINO-TERMINAL PROTEIN INTERACTION DOMAIN OF STAT-4 Summary

STATs are a family of transcription factors that are specifically triggered to participate in gene activation when cells encounter cytokines and growth factors. The crystal structure of a conserved domain comprising the first 123 residues of STAT-4 has been determined at 1.45 Å. This domain (N-terminal domain) has been implicated in several protein:protein interactions affecting transcription. The N-terminal domain enables STAT dimers to interact and to bind DNA cooperatively, a mechanism important for gene activation and binding site discrimination. The domain consists of 8 helices that are assembled into a hook-like structure. The crystal structure shows the formation of dimers formed by polar interactions across one face of the hook, revealing the nature of the cooperative interactions between spin STAT dimers. Mutagenesis of an invariant Trp residue that is at the heart of this interface abolishes cooperative dimer:dimer DNA binding by the full length protein in vitro and reduces transcriptional response after cytokine stimulation in vivo.

Materials and Methods

Expression and Purification of the STAT-4 N-terminal Domain:

The STAT-4 N-terminal domain was expressed as a C-terminal fusion with glutathione S-transferase (GST). The expression vector was constructed by PCR amplification (Vent DNA-polymerase, New England Biolabs) of the appropriate region of mouse STAT-4 cDNA and cloning of the fragment into the BamMI and EcoRI sites of pGEX2T (Pharmacia). Sequence comparison with the STAT-1 amino-terminal domain led to the decision to terminate the homologous domain in STAT-4 after residue 124. To facilitate cleavage of the GST tag from the recombinant STAT-4, three glycine residues were included after the thrombin cleavage site [Guan and Dixon, Anal. Biochem. 192:262 (1991)]. The resulting protein has four additional amino-terminal amino acids and Met 1 of STAT-4 is replaced with Gly. The construct was verified by dideoxy sequencing. BL21p(lysS) cells were grown and lysed as described [Vinkemeier et al., EMBO J. 15:5616 (1996)]. Soluble protein was incubated with 0.2 Vol. of a 50% (vol./vol.) slurry of glutathione agarose (Pharmacia). The bound protein was washed with cleavage buffer (50 mM Hepes/HCl pH 8.0, 150 mM KCl, 2.5 mM CaCl₂, 5 mM DTT). Cleavage with thrombin (˜1U/mg protein; Novagen) was performed overnight at room temperature with the protein bound to the beads. The eluted STAT-4 N-terminal domain fragment (in cleavage buffer) was concentrated to 20 mg/ml with centricon (Amicon) and used for crystallization.

Crystallization and Data Collection and Processing:

Initial studies were performed with the STAT-1 N-terminal domain, but led only to small, needle-like crystals. Hanging drops of one μl of the STAT-4 N-terminal domain fragment was mixed with equal volumes of reservoir buffer containing 0.2 M Na⁺CH₃COO⁻, 0.1 M Tris/HCL pH 8.0, 17% PEG4000. Hexagonal crystals (0.2×0.2×0.2 mm) were routinely grown overnight at 20° C. The crystals contain one molecule of the STAT-4 N-terminal domain in the asymmetric unit, and are in the space group P6₅22 (a=79.51 Å, b=79.51 Å, c=84.68 Å). Crystals were cryo-protected in reservoir solution enriched in PEG to 20% and glycerol to 22.5% prior to flash freezing. Heavy atom derivatives were prepared by soaking crystals for 30 minutes in a saturated solution of parahydroxy-mercuribenzoic acid diluted 1:20 with the cryoprotective solution. Data for the native crystal were collected at the Brookhaven National Laboratory at beam line X25 using a MAR imaging plate detector system. A MAD experiment on a parahydroxy-mercuribenzoic acid derivitized crystal was performed at the Brookhaven National Laboratory on beam line X4A using Fuji imaging plates. Data processing and reduction were carried out with HKL, DENZO, and SCALEPACK (written by Z. Otwinowski and W. Minor).

Model Building and Refinement.

Model building was performed using O [Jones et al., Acta Crystallogr. A47: 110-119 (1991)]. Bulk solvent correction and anisotropic B-factor scaling was applied during refinement using X-PLOR [Brünger, X-PLOR (Version 3.1) Manual, The Howard Hughes Medical Institute and Department of Molecular Biophysics and Biochemistry, Yale University, 260 Whitney Avenue, New Haven, Conn. 06511, (1992)]. The final refinement statistics are shown in Table 1. Of the five heterologous residues at the amino-terminus the first three residues (GlySerGly) as well as the C-terminal residue Gln 124 are not visible in the electron density map. No amino acids occupy disallowed regions of the Ramachandran plot and 95% fall into the most favored region.

Preparation, Expression, and Purification of the Mutated STAT-1α:

Mutated STAT-1α (Trp 37 to Ala) was expressed from pAcSG2 in baculovirus infected insect cells. PCR was used to exchange codon 37 [TGG] with [GCA] in the NcoI/SpeI fragment of human STAT-1 cDNA. Additionally, a 6-His tag was added to the C-terminus. Modifications were confirmed by sequencing. Insect cells were lysed (dounce homogenizer), mutated STAT-1α purified under native conditions on Ni²⁺ nitrilotriacetic acid (Qiagen) and eluted with 200 mM imidazole in 20 mM Tris/HCl, pH 8.0, 10 mM MgCl₂, 50 mM KCl, 5 mM DTT.

Preparation of EGF-receptor Kinase and In Vitro Phosphorylation of STAT Proteins.

Tyrosine phosphorylated wild type human STAT-1α was produced as described [Vinkemeier et al., EMBO J. 15:5616 (1996)]. In vitro phosphorylation was done as described [Vinkemeier et al., EMBO J. 15:5616 (1996)]. Human carcinoma A431 cells were grown to 90% confluency in 150 mm diameter plates in Dulbecco's modified Eagle's medium supplemented with 10% bovine calf serum (Hyclone). Cells were washed once with chilled PBS and lysates were prepared in 1 ml ice cold lysis buffer (10 Mm Hepes/HCl, 150 mM NaCl, 0.5% Triton X-100, 10% Glycerol, 1 mM Na₃VO₄, 10 mM EDTA, Complete™ protease inhibitors, pH 7.5). After 10 min on ice, the cells were scraped, vortexed and dounce homogenized (5 strokes). The lysates were cleared by centrifugation at 4° C. for 20 min at top speed in an Eppendorf microfuge and stored at −70° C. until needed. Immediately before use 1 volume of the lysate was diluted with 4 volumes of the lysis buffer (“diluted lysate”). EGF-receptor precipitates were obtained by incubating 5 ml of diluted lysate with 50 μg of an anti-EGF-receptor monoclonal antibody directed against the extracellular domain. After 2 hours of rotating the sample at 4° C., 750 μl of Protein-A-agarose (50% slurry; Oncogene Science) was added, and the incubation was allowed to proceed, while rotating, for another 1 hour. Agarose beads containing the EGF-receptor immunoprecipitates were then washed 5 times with lysis buffer and finally twice with storage buffer (20% Glycerol, 20 mM Hepes/HCl, 100 mM NaCl, 0.1 mM Na₃VO₄). Precipitates from 5 ml diluted lysate were dissolved in 0.5 ml storage buffer, flash frozen on dry ice and stored at −70° C. Immediately before an in vitro kinase reaction the Protein-A-agarose bound EGF-receptor from 5 ml dilute lysate was washed once with 1×kinase buffer (20 mM Tris/HCl, 50 mM KCl, 0.3 mM Na₃VO₄, 2 mM DTT, pH 8.0) and then dissolved in 0.4 ml (total volume) of this buffer. Afterwards the washed EGF-receptor precipitate was incubated on ice for 10 min in the presence of a final concentration of mouse EGF of 0.15 ng/μl. Phosphorylation reactions were carried out in Eppendorf tubes in a final volume of 1 ml. To the pre-incubated kinase preparation the following was added: 60 μl 10×kinase buffer, 20 μl 0.1 M DTT, 50 μl 0.1 M ATP, 4 mg STAT protein (Superdex 200 eluate for STAT1α and STAT1 β; ammonium sulfate pellets dissolved in [20 mM Tris/HCl, pH 8.0] for STAT1tc), 10 μl 1M MnCl₂ and dH₂O to 1 ml. The reaction was allowed to proceed for 15 hours at 4° C. After 3 hours an additional 15 μl of 0.1 M ATP was added.

Gel Shift Experiments and Determination of Tetramer Stability:

Gel shift experiments and determination of tetramer stability were done as described [Vinkemeier et al., EMBO J. 15:5616 (1996)] with an oligonucleotide containing two copies of the STAT recognition element from the c-fos gene [Wagner et al., EMBO J. 9:4477 (1990)] spaced by 10 base pairs (5′-GCCAGTCAGTTCCCGTCAATGCATCAGGTTCCCGTCAATGCAT-3′, SEQ ID NO:4). Both protein preparations (Tyr-phosphorylated wild type STAT-1α and W37A mutant) were titrated in gel shift experiments with an oligonucleotide containing a single M67 site (5′-GCCGATTTCCCGTAAATCAT-3′, SEQ ID NO:5) [Wagner et al., EMBO J. 9:4477 (1990)] to assure similar loading of active protein. A 12.5 μl reaction volume contained DNA binding buffer (20 mM Hepes/HCl, 4% Ficoll, 40 mM KCl, 10 mM MgCl₂, 10 mM CaCl₂, 1 mM DTT) radiolabelled DNA at a final concentration of 1×10⁻¹⁰ M unless stated otherwise, 50 ng dIdC, 0.2 mg/ml BSA (Boebringer Mannheim), and the indicated amount of purified phosphorylated STAT protein. The reaction volume was mixed and then incubated at room temperature. The time necessary to reach equilibrium was assessed by EMSA [(Stone et al., in Jost, J.P. & Saluz, H.P. (eds) A Laboratory Guide to In Vitro Studies of Protein-DNA Interactions BioMethods, vol.5:163-195 (1991)]. For all DNA fragments tested, equilibrium turned out to be fully established at the earliest timepoint that can be determined by this technique (30 sec). Therefore incubation periods of 5-15 minutes were chosen. Reaction products were loaded onto a 4% polyacrylamide gel (1.5 mm thick) containing 0.25×Tris-borate-EDTA which had been pre-run at 20V/cm for 2 hours at 4° C. Electrophoresis was continued for 60 minutes at 4° C. Gels were dried and exposed to X-ray film and quantitated by a Molecular Dynamics PhosphoImager.

Transient Transfections:

Transient transfections were performed on six well plates with 50% confluent U3A cells using the calcium phosphate method as instructed by the manufacturer (Stratagene) with the following modifications. Transfection reactions contained per well 4.5 μg of either wild type STAT-1α or the W37A mutant in plasmid pcDNA3 (Invitrogen), 4 μg luciferase reporter plasmid pLuc, and 0.4 μg β-galactosidase reporter plasmid (Stratagene). The luciferase reporter contained in its BamnH1 site as an enhancer element two tandemly arranged “weak” STAT-1 binding sites (5′-GATCAGTTCCCGTCAATCATGATCCAGTTCCCGTCAATGATCCCCGGGATC-3′, SEQ ID NO:6 binding sites underlined) from the human c-fos promoter. 36 hrs. after transfection, cells were treated with 5 ng/ml interferon-γ (Amgen) for 10 hrs. or left untreated. Luciferase assays were performed according to the manufacturer's protocol (Promega) and β-galactosidase assays were done as previously described [Ausubel et al., in Current Protocols in Molecular Biology, J. Wiley & Sons,Inc. (1994)]. Protein expression and Tyr-phosphorylation were checked in gel shift experiments with whole cell extracts for both wild type and mutant protein and were comparable. All results shown are luciferase activities normalized against the internal control β-galactosidase activity.

Results

Proteolytic digestion of purified STAT-1 showed that the N-terminal 131 residues form a stable domain that is readily cleaved off the intact molecule, indicating that it is an independently folded module [Vinkemeier et al., EMBO J. 15:5616 (1996)]. Sequence alignments show that the N-terminal domain is highly conserved in the STAT family of proteins (FIG. 1). The average sequence identity for this region between mammalian STAT proteins is 40%, and ranges from 51% between STAT-1 and STAT-4 to 20% between STAT-5 and STAT-6. Over the approximately 750 amino acids that span the length of the common core of the STATs only the SH2 domain is more highly conserved [Schindler and Darnell, Annu. Rev. Biochem. 64:621 (1995)]. The N-terminal domain is also found in the Drosophila STAT (dSTAT92E) [Hou et al., Cell 84:411 (1996); Yan et al., Cell 84:421 (1996)], (FIG. 1) and in the recently discovered STAT in Dictyostelium discoideum [Kawata et al., Cell 89:909 (1997)]. Interestingly, the first gene defect established in the DSTAT92 gene is a misspliced variant that produces both normal mRNA and an mRNA encoding only the N-terminal 41 residues. Expression of this fragment has a partial dominant negative effect on transcriptional activation by the wild type protein in cell culture. In the fly it is associated with a weak abnormal phenotype [Yan et al., Proc. Natl. Acad. Sci. USA 93:5842 (1996)].

As disclosed herein the crystal structure of the N-terminal domain of STAT-4 has been determined by multi-wavelength anomalous diffraction and refined at a resolution of 1.45 Å (R=19.4%, R_(free)=22.3%) (Table 1). The STAT-4 N-domain is all helical, with an unusual architecture. Instead of the up-down connectivity of helix bundles or the box-like helical packing of the globin fold, the N-domain is constructed from three distinct structural elements that pack together. The N-terminal 40 residues encompass the first 4 helices (α1-α4), which form a ring-shaped element (colored red in FIGS. 2 and 3B). A small helix (α5) connects this ring to the next structural element, an anti-parallel coiled-coil formed by helices α6 and α7. The heptad repeat of hydrophobic amino acids, characteristic of coiled-coils, is conserved across the STATs (FIG. 1). Finally, the distal surface of the ring-shaped element forms a docking site for the last helix in the structure (α8). The overall appearance of the structure is that of a triangular hook, with the inner surface of the hook being formed by the intersection of the proximal surfaces of the ring-shaped element and the coiled-coil. The N-domain of STAT-4 is dimeric in solution, and a two-fold symmetry axis in the crystal generates a dimer with an extensive polar interface that involves one face of the hook.

The N-domain (as depicted below with the amino acid numbering of SEQ ID NO:9) has a well defined hydrophobic core that is conserved across the STATs, consistent with a stable and defined fold (FIGS. 1 and 2). However, a notable feature of the N-terminal ring-shaped element is that it is stabilized by polar interactions involving buried charges. The ring is closed off by a helix-dipole interaction between the N-terminal region of helix α1 and the carboxylate group of Glu 39, presented by the C-terminal region of helix α4 (FIG. 2B). Glu 39 forms a hydrogen bond with the amide nitrogen of the Gln 3 residue, and is oriented correctly for this charge dipole interaction by the side chain of Arg 31, which in turn forms a buried ion pair with Glu 112. Glu 112 is positioned by interactions with Tyr 22 and a buried water molecule. Each of the side chains involved is invariant in all STATs (FIG. 1), indicating that the ring-shaped element is conserved in the STAT architecture.

TABLE 1 Summary of the Crystallographic Analysis Resolution Reflections Completeness R_(sym)* Sites (Å) total/unique (%) (%) I_(n)/σ† (N) FOM # Native Data 1.45 237647/27594 96.4 (91.7) 7.2 (19.2) 19.6 (4.2) MAD Analysis λ1 = 1.00842Å 2.15 112445/8854  98.6 (95.7) 7.7 (21.2) 24.7 (6.1) 2 λ2 = 1.01337Å 2.15 131403/8992  100.0 (100.0) 7.3 (21.1) 27.2 (8.1) 2 λ3 = 1.00932Å 2.00 126292/10731 96.6 (95.7) 8.4 (33.4) 22.1 (4.5) 2 0.542 Completeness R-factor R-free Refinement Cut Off Reflections (%) (%) ¶ (%) ‡ 10-1.45Å |F|/σ(|F|) > 2 25285 88.8 18.8 21.6 10-1.45Å all data 26560 93.3 19.4 22.3 r.m.s. deviation of model bond lengths (Å) 0.015 bond angles (degrees) 1.4 average B-factor (protein) (Å²) 9.6 water molecules 165 average B-factor (water) (Å²) 25 *R_(sym) = 100 × Σ_(h)Σ_(i)I_(h,i) − I_(h)|/Σ_(h)Σ_(i)I_(h,i) for the intensity of i observations of reflection h. For R_(sym) and completeness, the numbers in parenthesis refer to data in the shell of highest resolution. I_(n) is the mean intensity of the reflection. † I_(n)/σ = mean intensity/mean standard deviation. # FOM = overall mean figure of merit [|ΣP(α)e^(iα)/ΣP(α)|], where α is the phase and P(α) is the phase distribution probability. ¶ R-factor = 100 × Σ|F_(obs) − F_(calc)|/Σ|F_(obs)|, where F_(obs) and F_(calc) are the observed and calculated structure factors, respectively. ‡ R-free = is the same as R-factor, but calculated on the 5% of the reflection data excluded from refinement.

TABLE II Interactions Between the Two N-terminal Domains of Stat4 (SEQ ID NO:9) Molecule A Å Water A Å Water B Å Water C Å Molecule B sidechain-sidechain contacts; direct Asn5 3.0 Asn5 Trp37 3.2 Glu66 Thr40 3.2 Arg70 3.2 Gln67 Gln41 2.9 Glu66 Asp42 3.3 Lys73 Gln63 3.6 Gln63 Glu66 3.2 Trp37 Gln67 3.2 Thr40 Arg70 3.0 Thr40 Lys73 3.3 Asp42 sidechain-sidechain contacts; water mediated Gln36 3.0 H₂O 984 2.9 H₂O 269 2.7 Glu39 Trp37 3.1 H₂O 167 3.0 Gln63 Glu39 2.8 H₂O 186 3.4 Arg70 2.7 H₂O 269 2.8 Arg70 Thr40 2.8 H₂O 314 2.8 Gln67 2.8 H₂O 314 2.8 H₂O 6 2.8 H₂O 314 2.8 Thr40 2.8 Glu66 Gln41 2.8 H₂O 5212 2.9 Glu66 Asn59 2.9 H₂O 212 2.9 Glu66 Gln63 2.7 H₂O 901 2.7 Gln63 3.0 H₂O 164 3.1 Trp37 Glu66 2.7 H₂O 164 3.1 Trp37 2.8 H₂O 6 2.8 H₂O 314 2.8 Thr40 2.9 H₂O 212 2.9 Asn59 Gln67 3.2 H₂O 6 2.8 H₂O 314 2.8 Thr40 Arg70 2.8 H₂O 314 2.8 Thr40 3.4 H₂O 186 2.8 Glu39 3.1 H₂O 269 2.7 Glu39 sidechain-mainchain contacts; direct Gln36-N 2.9 Gln36 Glu39-O 3.6 Arg70 Arg70 2.6 Glu39-O uz,9/30 sidechain-mainchain contacts; water mediated His32-O 2.9 H₂O 1024 3.1 Gln36 Gln36-N 3.2 H₂O 41 2.7 Gln36 Gln36 3.1 H₂O 1024 2.9 His31-O Trp37-N 2.9 H₂O 41 2.7 Gln36 Glu66-O 2.7 H₂O 5107 2.8 H₂O 5078 2.7 Gln41 The distance in Ångstroms (Å) between the interacting amino acid residues and/or molecules are provided. The water molecules are numbered according to their structure file. The mainchain contacts are indicated by “-N” or “-O” for backbone amido or carboxyl groups respectively.

A consequence of the utilization of these polar groups in the ring-shaped element is the formation of a compact and potentially specific interaction surface. This structural element forms a relatively flat molecular surface that packs at an angle against another surface presented by the coiled-coil formed by helices α6 and α7. The juxta positioning of the surface of the ring-shaped element with that of the coiled-coil results in a wedge-shaped groove. This. groove is lined with hydrophobic residues, with polar residues at the center, and has the natural appearance of a ligand binding site. A possible function for this groove is suggested by the fact that replacement of Arg 31 or Glu 39 in STAT-1 (see SEQ ID NO:7) by Ala results in a molecule that is much slower to dephosphorylate after interferon-γ induction when compared to wild type protein [Shuai et al., Mol. Cell. Biol. 16:4932 (1996)]. Mutation of these two residues is likely to drastically alter the conformation of the ring-shaped element (FIG. 2B), suggesting that a phosphatase that controls STAT dephosphorylation might bind to the groove in the N-terminal domain.

There is one molecule in the asymmetric unit of the STAT-4 crystal, and it is related to another by a two-fold symmetry axis (FIGS. 2A and 3A). There is an extensive interface between the two monomers of the dimer, burying 1,714 Å² of surface area (FIG. 3A). An extended intermolecular hydrogen bonding network is formed at the interface, involving 15 amino acid side chains and 12 water molecules per monomer (FIG. 3B). In addition 5 backbone contacts are also observed in each monomer. 11 of the 15 residues at the interface make direct hydrogen bonding contacts to the other monomer. The water molecules at the interface are very well defined in the electron density map, many of them having low temperature factors (<10 Å²) (FIG. 3C).

The interface is almost entirely polar, with no involvement of hydrophobic side chains. In contrast to the leucine zipper, wherein hydrophobic residues are utilized to generate the intermolecular interface by the formation of a coiled-coil across the dimer interface [Lupas, Trends in Biochemical Sciences 21:375 (1996)], the coiled-coil in the N-termninal domain is firmly anchored within one N-terminal domain and its role is to serve as an architectural support for the presentation of a number of interacting side chains at the dimer interface and at the potential inteaction groove. While hydrophobic interactions are associated with stabilization of folded protein structures and are often found at the core of tight interfaces, polar interactions can provide both stability and specificity in protein-protein interactions [Fersht et al., Nature 314:235 (1985); Xu et al., Journal of Molecular Biology 265:68 (1997)]. In contrast to the residues that constitute the buried core of the N-terminal domain, which are conserved across STATs, the majority of the residues at the dimer interface are not conserved (FIGS. 1 and 3B). This variation could provide specificity in STAT dimer:dimer interactions on DNA. Only two of the residues at the interface are invariant in all STATs (SEQ ID NOs:2 and 7-13, and see, FIG. 1B): Trp 37, a central anchor residue at the interface, and Glu 39, which also participates in the formation of the ring-shaped element.

To test the physiological relevance of the dimer that is observed in the crystal structure it was determined if the mutation of the critical Trp 37 residue at the interface would disrupt or reduce oligomerization in vitro and transcriptional activation in vivo. These experiments were carried out using STAT-1, for which a DNA-protein interaction assay had been worked out previously [Vinkemeier et al., EMBO J. 15:5616 (1996)]. In addition, a cell line (U3A) lacking STAT-1 is available [Müller et al., EMBO J. 12:4221 (1993)] allowing the introduction of expression vectors encoding mutant STAT-1 molecules. The close similarity between the N-terminal domains of STAT-1 and STAT-4 (51% amino acid identity) ensures that the structural information derived from the STAT-4 crystal structure is an accurate representation of the STAT-1 architecture.

Conditions under which an oligonucleotide bearing tandem binding sites binds full length STAT-1 as a tetramer have been established; the site contains two “weak” binding sites, spaced 10 base pairs apart [Vinkemeier et al., EMBO J. 15:5616 (1996)]. Competition experiments show that the off time is long for wild type STAT-1 (greater than 15-30 minutes), indicating the formation of a stable complex. In contrast, if an oligonucleotide containing only one “weak” site is used instead, the off time was less than 30 seconds [Vinkemeier et al., EMBO J. 15:5616 (1996)]. The stabilization of STAT-1 on oligonucleotides containing tandem binding sites is not observed if the N-terminal domain is deleted [Vinkemejer et al., EMBO J. 15;5616 (1996)]. This assay was used to test the DNA binding properties of STAT-1 in which Trp 37 was replaced by Ala (W37A). Trp 37 plays a central role at the dimer interface, and participates in both direct and water mediated interactions with the other monomer (FIG. 3C).

The W37A mutant protein binds to the DNA probe with tandem sites, but the interaction is completely displaced by the addition of unlabelled oligonucleotides (FIG. 4A). In contrast, the wild-type protein is resistant to displacement for more than 15 minutes. The same two tandem “weak” binding sites were used to drive transcription from a reporter gene in an interferon-dependent transcriptional assay. U3A cells were co-transfected with the reporter gene and with either wild type or W37A mutant STAT-1. The rather weak transcriptional induction by interferon γ (approximately 2-fold) was abolished by the mutation (FIG. 4B).

Activated and dimeric STAT proteins do not form detectable tetrameres in solution in the absence of DNA. It is not known whether this is a consequence of limited binding affinity between N-terminal domains or whether the conformation of the STAT molecule in the absence of DNA impedes further oligomerization. In any case, the presentation of highly polar and unique interaction surfaces by the N-terminal domains of the STATs provides a ready means for generating very specific interactions between adjacent STAT dimers on the DNA, since the hydrogen bonding constraints of the interacting groups places stereochemical constraints on potential partners. While each N-domain dimer is closed, the fact that each STAT dimer presents two N-domains for interaction makes possible the generation of open ended STAT-STAT interactions that are limited only by the nature and number of the adjacent DNA binding sites.

The present invention is not to be limited in scope by the specific embodiments describe herein. Indeed, various modifications of the invention in addition to those described herein will become apparent to those skilled in the art from the foregoing description and the accompanying figures. Such modifications are intended to fall within the scope of the appended claims.

It is further to be understood that all base sizes or amino acid sizes, and all molecular weight or molecular mass values, given for nucleic acids or polypeptides are approximate, and are provided for description.

Various publications are cited herein, the disclosures of which are incorporated by reference in their entireties. The citation of any reference herein should not be construed as an admission that such reference is available as “Prior Art” to the instant application.

13 13 amino acids amino acid single linear peptide NO not provided 1 Arg Xaa Xaa Leu Xaa Xaa Trp Xaa Glu Xaa Gln Xaa Trp 1 5 10 851 amino acids amino acid single linear protein NO not provided 2 Met Ala Gln Trp Glu Met Leu Gln Asn Leu Asp Ser Pro Phe Gln Asp 1 5 10 15 Gln Leu His Gln Leu Tyr Ser His Ser Leu Leu Pro Val Asp Ile Arg 20 25 30 Gln Tyr Leu Ala Val Trp Ile Glu Asp Gln Asn Trp Gln Glu Ala Ala 35 40 45 Leu Gly Ser Asp Asp Ser Lys Ala Thr Met Leu Phe Phe His Phe Leu 50 55 60 Asp Gln Leu Asn Tyr Glu Cys Gly Arg Cys Ser Gln Asp Pro Glu Ser 65 70 75 80 Leu Leu Leu Gln His Asn Leu Arg Lys Phe Cys Arg Asp Ile Gln Pro 85 90 95 Phe Ser Gln Asp Pro Thr Gln Leu Ala Glu Met Ile Phe Asn Leu Leu 100 105 110 Leu Glu Glu Lys Arg Ile Leu Ile Gln Ala Gln Arg Ala Gln Leu Glu 115 120 125 Gln Gly Glu Pro Val Leu Glu Thr Pro Val Glu Ser Gln Gln His Glu 130 135 140 Ile Glu Ser Arg Ile Leu Asp Leu Arg Ala Met Met Glu Lys Leu Val 145 150 155 160 Lys Ser Ile Ser Gln Leu Lys Asp Gln Gln Asp Val Phe Cys Phe Arg 165 170 175 Tyr Lys Ile Gln Ala Lys Gly Lys Thr Pro Ser Leu Asp Pro His Gln 180 185 190 Thr Lys Glu Gln Lys Ile Leu Gln Glu Thr Leu Asn Glu Leu Asp Lys 195 200 205 Arg Arg Lys Glu Val Leu Asp Ala Ser Lys Ala Leu Leu Gly Arg Leu 210 215 220 Thr Thr Leu Ile Glu Leu Leu Leu Pro Lys Leu Glu Glu Trp Lys Ala 225 230 235 240 Gln Gln Gln Lys Ala Cys Ile Arg Ala Pro Ile Asp His Gly Leu Glu 245 250 255 Gln Leu Glu Thr Trp Phe Thr Ala Gly Ala Lys Leu Leu Phe His Leu 260 265 270 Arg Gln Leu Leu Lys Glu Leu Lys Gly Leu Ser Cys Leu Val Ser Tyr 275 280 285 Gln Asp Asp Pro Leu Thr Lys Gly Val Asp Leu Arg Asn Ala Gln Val 290 295 300 Thr Glu Leu Leu Gln Arg Leu Leu His Arg Ala Phe Val Val Glu Thr 305 310 315 320 Gln Pro Cys Met Pro Gln Thr Pro His Arg Pro Leu Ile Leu Lys Thr 325 330 335 Gly Ser Lys Phe Thr Val Arg Thr Arg Leu Leu Val Arg Leu Gln Glu 340 345 350 Gly Asn Glu Ser Leu Thr Val Glu Val Ser Ile Asp Arg Asn Pro Pro 355 360 365 Gln Leu Gln Gly Phe Arg Lys Phe Asn Ile Leu Thr Ser Asn Gln Lys 370 375 380 Thr Leu Thr Pro Glu Lys Gly Gln Ser Gln Gly Leu Ile Trp Asp Phe 385 390 395 400 Gly Tyr Leu Thr Leu Val Glu Gln Arg Ser Gly Gly Ser Gly Lys Gly 405 410 415 Ser Asn Lys Gly Pro Leu Gly Val Thr Glu Glu Leu His Ile Ile Ser 420 425 430 Phe Thr Val Lys Tyr Thr Tyr Gln Gly Leu Lys Gln Glu Leu Lys Thr 435 440 445 Asp Thr Leu Pro Val Val Ile Ile Ser Asn Met Asn Gln Leu Ser Ile 450 455 460 Ala Trp Ala Ser Val Leu Trp Phe Asn Leu Leu Ser Pro Asn Leu Gln 465 470 475 480 Asn Gln Gln Phe Phe Ser Asn Pro Pro Lys Ala Pro Trp Ser Leu Leu 485 490 495 Gly Pro Ala Leu Ser Trp Gln Phe Ser Ser Tyr Val Gly Arg Gly Leu 500 505 510 Asn Ser Asp Gln Leu Ser Met Leu Arg Asn Lys Leu Phe Gly Gln Asn 515 520 525 Cys Arg Thr Glu Asp Pro Leu Leu Ser Trp Ala Asp Phe Thr Lys Arg 530 535 540 Glu Ser Pro Pro Gly Lys Leu Pro Phe Trp Thr Trp Leu Asp Lys Ile 545 550 555 560 Leu Glu Leu Val His Asp His Leu Lys Asp Leu Trp Asn Asp Gly Arg 565 570 575 Ile Met Gly Phe Val Ser Arg Ser Gln Glu Arg Arg Leu Leu Lys Lys 580 585 590 Thr Met Ser Gly Thr Phe Leu Leu Arg Phe Ser Glu Ser Ser Glu Gly 595 600 605 Gly Ile Thr Cys Ser Trp Val Glu His Gln Asp Asp Asp Lys Val Leu 610 615 620 Ile Tyr Ser Val Gln Pro Tyr Thr Lys Glu Val Leu Gln Ser Leu Pro 625 630 635 640 Leu Thr Glu Ile Ile Arg His Tyr Gln Leu Leu Thr Glu Glu Asn Ile 645 650 655 Pro Glu Asn Pro Leu Arg Phe Leu Tyr Pro Arg Ile Pro Arg Asp Glu 660 665 670 Ala Phe Gly Cys Tyr Tyr Gln Glu Lys Val Asn Leu Gln Glu Arg Arg 675 680 685 Lys Tyr Leu Lys His Arg Leu Ile Val Val Ser Asn Arg Gln Val Asp 690 695 700 Glu Leu Gln Gln Pro Leu Glu Leu Lys Pro Glu Pro Glu Leu Glu Ser 705 710 715 720 Leu Glu Leu Glu Leu Gly Leu Val Pro Glu Pro Glu Leu Ser Leu Asp 725 730 735 Leu Glu Pro Leu Leu Lys Ala Gly Leu Asp Leu Gly Pro Glu Leu Glu 740 745 750 Ser Val Leu Glu Ser Thr Leu Glu Pro Val Ile Glu Pro Thr Leu Cys 755 760 765 Met Val Ser Gln Thr Val Pro Glu Pro Asp Gln Gly Pro Val Ser Gln 770 775 780 Pro Val Pro Glu Pro Asp Leu Pro Cys Asp Leu Arg His Leu Asn Thr 785 790 795 800 Glu Pro Met Glu Ile Phe Arg Asn Cys Val Lys Ile Glu Glu Ile Met 805 810 815 Pro Asn Gly Asp Pro Leu Leu Ala Gly Gln Asn Thr Val Asp Glu Val 820 825 830 Tyr Val Ser Arg Pro Ser His Phe Tyr Thr Asp Gly Pro Leu Met Pro 835 840 845 Ser Asp Phe 850 5 amino acids amino acid single linear peptide NO not provided 3 Gly Ser Gly Gly Gly 1 5 43 base pairs nucleic acid single linear other nucleic acid /desc = “PRIMER” NO not provided 4 GCCAGTCAGT TCCCGTCAAT GCATCAGGTT CCCGTCAATG CAT 43 20 base pairs nucleic acid single linear other nucleic acid /desc = “PRIMER” NO not provided 5 GCCGATTTCC CGTAAATCAT 20 52 base pairs nucleic acid single linear other nucleic acid /desc = “PRIMER” NO not provided 6 GATCAGTTCC CGTCAATCNA TGATCCAGTT CCCGTCAATG ATCCCCGGGA TC 52 749 amino acids amino acid single linear protein NO not provided 7 Met Ser Gln Trp Phe Glu Leu Gln Gln Leu Asp Ser Lys Phe Leu Glu 1 5 10 15 Gln Val His Gln Leu Tyr Asp Asp Ser Phe Pro Met Glu Ile Arg Gln 20 25 30 Tyr Leu Ala Gln Trp Leu Glu Lys Gln Asp Trp Glu His Ala Ala Tyr 35 40 45 Asp Val Ser Phe Ala Thr Ile Arg Phe His Asp Leu Leu Ser Gln Leu 50 55 60 Asp Asp Gln Tyr Ser Arg Phe Ser Leu Glu Asn Asn Phe Leu Leu Gln 65 70 75 80 His Asn Ile Arg Lys Ser Lys Arg Asn Leu Gln Asp Asn Phe Gln Glu 85 90 95 Asp Pro Val Gln Met Ser Met Ile Ile Tyr Asn Cys Leu Lys Glu Glu 100 105 110 Arg Lys Ile Leu Glu Asn Ala Gln Arg Phe Asn Gln Ala Gln Glu Gly 115 120 125 Asn Ile Gln Asn Thr Val Met Leu Asp Lys Gln Lys Glu Leu Asp Ser 130 135 140 Lys Val Arg Asn Val Lys Asp Gln Val Met Cys Ile Glu Gln Glu Ile 145 150 155 160 Lys Thr Leu Glu Glu Leu Gln Asp Glu Tyr Asp Phe Lys Cys Lys Thr 165 170 175 Ser Gln Asn Arg Glu Gly Glu Ala Asn Gly Val Ala Lys Ser Asp Gln 180 185 190 Lys Gln Glu Gln Leu Leu Leu His Lys Met Phe Leu Met Leu Asp Asn 195 200 205 Lys Arg Lys Glu Ile Ile His Lys Ile Arg Glu Leu Leu Asn Ser Ile 210 215 220 Glu Leu Thr Gln Asn Thr Leu Ile Asn Asp Glu Leu Val Glu Trp Lys 225 230 235 240 Arg Arg Gln Gln Ser Ala Cys Ile Gly Gly Pro Pro Asn Ala Cys Leu 245 250 255 Asp Gln Leu Gln Thr Trp Phe Thr Ile Val Ala Glu Thr Leu Gln Gln 260 265 270 Ile Arg Gln Gln Leu Lys Lys Leu Glu Glu Leu Glu Gln Lys Phe Thr 275 280 285 Tyr Glu Pro Asp Pro Ile Thr Lys Asn Lys Gln Val Leu Ser Asp Arg 290 295 300 Thr Phe Leu Leu Phe Gln Gln Leu Ile Gln Ser Ser Phe Val Val Glu 305 310 315 320 Arg Gln Pro Cys Met Pro Thr His Pro Gln Arg Pro Leu Val Leu Lys 325 330 335 Thr Gly Val Gln Phe Thr Val Lys Ser Arg Leu Leu Val Lys Leu Gln 340 345 350 Glu Ser Asn Leu Leu Thr Lys Val Lys Cys His Phe Asp Lys Asp Val 355 360 365 Asn Glu Lys Asn Thr Val Lys Gly Phe Arg Lys Phe Asn Ile Leu Gly 370 375 380 Thr His Thr Lys Val Met Asn Met Glu Glu Ser Thr Asn Gly Ser Leu 385 390 395 400 Ala Ala Glu Leu Arg His Leu Gln Leu Lys Glu Gln Lys Asn Ala Gly 405 410 415 Asn Arg Thr Asn Glu Gly Pro Leu Ile Val Thr Glu Glu Leu His Ser 420 425 430 Leu Ser Phe Glu Thr Gln Leu Cys Gln Pro Gly Leu Val Ile Asp Leu 435 440 445 Glu Thr Thr Ser Leu Pro Val Val Val Ile Ser Asn Val Ser Gln Leu 450 455 460 Pro Ser Gly Trp Ala Ser Ile Leu Trp Tyr Asn Met Leu Val Thr Glu 465 470 475 480 Pro Arg Asn Leu Ser Phe Phe Leu Asn Pro Pro Cys Ala Trp Trp Ser 485 490 495 Gln Leu Ser Glu Val Leu Ser Trp Gln Phe Ser Ser Val Thr Lys Arg 500 505 510 Gly Leu Asn Ala Asp Gln Leu Ser Met Leu Gly Glu Lys Leu Leu Gly 515 520 525 Pro Asn Ala Gly Pro Asp Gly Leu Ile Pro Trp Thr Arg Phe Cys Lys 530 535 540 Glu Asn Ile Asn Asp Lys Asn Phe Ser Phe Trp Pro Trp Ile Asp Thr 545 550 555 560 Ile Leu Glu Leu Ile Lys Asn Asp Leu Leu Cys Leu Trp Asn Asp Gly 565 570 575 Cys Ile Met Gly Phe Ile Ser Lys Glu Arg Glu Arg Ala Leu Leu Lys 580 585 590 Asp Gln Gln Pro Gly Thr Phe Leu Leu Arg Phe Ser Glu Ser Ser Arg 595 600 605 Glu Gly Ala Ile Thr Phe Thr Trp Val Glu Arg Ser Gln Asn Gly Gly 610 615 620 Glu Pro Asp Phe His Ala Val Glu Pro Tyr Thr Lys Lys Glu Leu Ser 625 630 635 640 Ala Val Thr Phe Pro Asp Ile Ile Arg Asn Tyr Lys Val Met Ala Ala 645 650 655 Glu Asn Ile Pro Glu Asn Pro Leu Lys Tyr Leu Tyr Pro Asn Ile Asp 660 665 670 Lys Asp His Ala Phe Gly Lys Tyr Tyr Ser Arg Pro Lys Glu Ala Pro 675 680 685 Glu Pro Met Glu Leu Asp Asp Pro Lys Arg Thr Gly Tyr Ile Lys Thr 690 695 700 Glu Leu Ile Ser Val Ser Glu Val His Pro Ser Arg Leu Gln Thr Thr 705 710 715 720 Asp Asn Leu Leu Pro Met Ser Pro Glu Glu Phe Asp Glu Met Ser Arg 725 730 735 Ile Val Gly Pro Glu Phe Asp Ser Met Met Ser Thr Val 740 745 770 amino acids amino acid single linear protein NO not provided 8 Met Ala Gln Trp Asn Gln Leu Gln Gln Leu Asp Thr Arg Tyr Leu Lys 1 5 10 15 Gln Leu His Gln Leu Tyr Ser Asp Thr Phe Pro Met Glu Leu Arg Gln 20 25 30 Phe Leu Ala Pro Trp Ile Glu Ser Gln Asp Trp Ala Tyr Ala Ala Ser 35 40 45 Lys Glu Ser His Ala Thr Leu Val Phe His Asn Leu Leu Gly Glu Ile 50 55 60 Asp Gln Gln Tyr Ser Arg Phe Leu Gln Glu Ser Asn Val Leu Tyr Gln 65 70 75 80 His Asn Leu Arg Arg Ile Lys Gln Phe Leu Gln Ser Arg Tyr Leu Glu 85 90 95 Lys Pro Met Glu Ile Ala Arg Ile Val Ala Arg Cys Leu Trp Glu Glu 100 105 110 Ser Arg Leu Leu Gln Thr Ala Ala Thr Ala Ala Gln Gln Gly Gly Gln 115 120 125 Ala Asn His Pro Thr Ala Ala Val Val Thr Glu Lys Gln Gln Met Leu 130 135 140 Glu Gln His Leu Gln Asp Val Arg Lys Arg Val Gln Asp Leu Glu Gln 145 150 155 160 Lys Met Lys Val Val Glu Asn Leu Gln Asp Asp Phe Asp Phe Asn Tyr 165 170 175 Lys Thr Leu Lys Ser Gln Gly Asp Met Gln Asp Leu Asn Gly Asn Asn 180 185 190 Gln Ser Val Thr Arg Gln Lys Met Gln Gln Leu Glu Gln Met Leu Thr 195 200 205 Ala Leu Asp Gln Met Arg Arg Ser Ile Val Ser Glu Leu Ala Gly Leu 210 215 220 Leu Ser Ala Met Glu Tyr Val Gln Lys Thr Leu Thr Asp Glu Glu Leu 225 230 235 240 Ala Asp Trp Lys Arg Arg Gln Gln Ile Ala Cys Ile Gly Gly Pro Pro 245 250 255 Asn Ile Cys Leu Asp Arg Leu Glu Asn Trp Ile Thr Ser Leu Ala Glu 260 265 270 Ser Gln Leu Gln Thr Arg Gln Gln Ile Lys Lys Leu Glu Glu Leu Gln 275 280 285 Gln Lys Val Ser Tyr Lys Gly Asp Pro Ile Val Gln His Arg Pro Met 290 295 300 Leu Glu Glu Arg Ile Val Glu Leu Phe Arg Asn Leu Met Lys Ser Ala 305 310 315 320 Phe Val Val Glu Arg Gln Pro Cys Met Pro Met His Pro Asp Arg Pro 325 330 335 Leu Val Ile Lys Thr Gly Val Gln Phe Thr Thr Lys Val Arg Leu Leu 340 345 350 Val Lys Phe Pro Glu Leu Asn Tyr Gln Leu Lys Ile Lys Val Cys Ile 355 360 365 Asp Lys Asp Ser Gly Asp Val Ala Ala Leu Arg Gly Ser Arg Lys Phe 370 375 380 Asn Ile Leu Gly Thr Asn Thr Lys Val Met Asn Met Glu Glu Ser Asn 385 390 395 400 Asn Gly Ser Leu Ser Ala Glu Phe Lys His Leu Thr Leu Arg Glu Gln 405 410 415 Arg Cys Gly Asn Gly Gly Arg Ala Asn Cys Asp Ala Ser Leu Ile Val 420 425 430 Thr Glu Glu Leu His Leu Ile Thr Phe Glu Thr Glu Val Tyr His Gln 435 440 445 Gly Leu Lys Ile Asp Leu Glu Thr His Ser Leu Pro Val Val Val Ile 450 455 460 Ser Asn Ile Cys Gln Met Pro Asn Ala Trp Ala Ser Ile Leu Trp Tyr 465 470 475 480 Asn Met Leu Thr Asn Asn Pro Lys Asn Val Asn Phe Phe Thr Lys Pro 485 490 495 Pro Ile Gly Thr Trp Asp Gln Val Ala Glu Val Leu Ser Trp Gln Phe 500 505 510 Ser Ser Thr Thr Lys Arg Gly Leu Ser Ile Glu Gln Leu Thr Thr Leu 515 520 525 Ala Glu Lys Leu Leu Gly Pro Gly Val Asn Tyr Ser Gly Cys Gln Ile 530 535 540 Thr Trp Ala Lys Phe Cys Lys Glu Asn Met Ala Gly Lys Gly Phe Ser 545 550 555 560 Phe Trp Val Trp Leu Asp Asn Ile Ile Asp Leu Val Lys Lys Tyr Ile 565 570 575 Leu Ala Leu Trp Asn Glu Gly Tyr Ile Met Gly Phe Ile Ser Lys Glu 580 585 590 Arg Glu Arg Ala Ile Leu Ser Thr Lys Pro Pro Gly Thr Phe Leu Leu 595 600 605 Arg Phe Ser Glu Ser Ser Lys Glu Gly Gly Val Thr Phe Thr Trp Val 610 615 620 Glu Lys Asp Ile Ser Gly Lys Thr Gln Ile Gln Ser Val Glu Pro Tyr 625 630 635 640 Thr Lys Gln Gln Leu Asn Asn Met Ser Phe Ala Glu Ile Ile Met Gly 645 650 655 Tyr Lys Ile Met Asp Ala Thr Asn Ile Leu Val Ser Pro Leu Val Tyr 660 665 670 Leu Tyr Pro Asp Ile Pro Lys Glu Glu Ala Phe Gly Lys Tyr Cys Arg 675 680 685 Pro Glu Ser Gln Glu His Pro Glu Ala Asp Pro Gly Ser Ala Ala Pro 690 695 700 Tyr Leu Lys Thr Lys Phe Ile Cys Val Thr Pro Thr Thr Cys Ser Asn 705 710 715 720 Thr Ile Asp Leu Pro Met Ser Pro Arg Thr Leu Asp Ser Leu Met Gln 725 730 735 Phe Gly Asn Asn Gly Glu Gly Ala Glu Pro Ser Ala Gly Gly Gln Phe 740 745 750 Glu Ser Leu Thr Phe Asp Met Asp Leu Thr Ser Glu Cys Ala Thr Ser 755 760 765 Pro Met 770 749 amino acids amino acid single linear protein NO not provided 9 Met Ser Gln Trp Asn Gln Val Gln Gln Leu Glu Ile Lys Phe Leu Glu 1 5 10 15 Gln Val Asp Gln Phe Tyr Asp Asp Asn Phe Pro Met Glu Ile Arg His 20 25 30 Leu Leu Ala Gln Trp Ile Glu Thr Gln Asp Trp Glu Val Ala Ser Asn 35 40 45 Asn Glu Thr Met Ala Thr Ile Leu Leu Gln Asn Leu Leu Ile Gln Leu 50 55 60 Asp Glu Gln Leu Gly Arg Val Ser Lys Glu Lys Asn Leu Leu Leu Ile 65 70 75 80 His Asn Leu Lys Arg Ile Arg Lys Val Leu Gln Gly Lys Phe His Gly 85 90 95 Asn Pro Met His Val Ala Val Val Ile Ser Asn Cys Leu Arg Glu Glu 100 105 110 Arg Arg Ile Leu Ala Ala Ala Asn Met Pro Ile Gln Gly Pro Leu Glu 115 120 125 Lys Ser Leu Gln Ser Ser Ser Val Ser Glu Arg Gln Arg Asn Val Glu 130 135 140 His Lys Val Ser Ala Ile Lys Asn Ser Val Gln Met Thr Glu Gln Asp 145 150 155 160 Thr Lys Tyr Leu Glu Asp Leu Gln Asp Glu Phe Asp Tyr Arg Tyr Lys 165 170 175 Thr Ile Gln Thr Met Asp Gln Gly Asp Lys Asn Ser Ile Leu Val Asn 180 185 190 Gln Glu Val Leu Thr Leu Leu Gln Glu Met Leu Asn Ser Leu Asp Phe 195 200 205 Lys Arg Lys Glu Ala Leu Ser Lys Met Thr Gln Ile Val Asn Glu Thr 210 215 220 Asp Leu Leu Met Asn Ser Met Leu Leu Glu Glu Leu Gln Asp Trp Lys 225 230 235 240 Lys Arg Gln Gln Ile Ala Cys Ile Gly Gly Pro Leu His Asn Gly Leu 245 250 255 Asp Gln Leu Gln Asn Cys Phe Thr Leu Leu Ala Glu Ser Leu Phe Gln 260 265 270 Leu Arg Gln Gln Leu Glu Lys Leu Gln Glu Gln Ser Thr Lys Met Thr 275 280 285 Tyr Glu Gly Asp Pro Ile Pro Ala Gln Arg Ala His Leu Leu Glu Arg 290 295 300 Ala Thr Phe Leu Ile Tyr Asn Leu Phe Lys Asn Ser Phe Val Val Glu 305 310 315 320 Arg Gln Pro Cys Met Pro Thr His Pro Gln Arg Pro Met Val Leu Lys 325 330 335 Thr Leu Ile Gln Phe Thr Val Lys Leu Arg Leu Leu Ile Lys Leu Pro 340 345 350 Glu Leu Asn Tyr Gln Val Lys Val Lys Ala Ser Ile Asp Lys Asn Val 355 360 365 Ser Thr Leu Ser Asn Arg Arg Phe Val Leu Cys Gly Thr His Val Lys 370 375 380 Ala Met Ser Ser Glu Glu Ser Ser Asn Gly Ser Leu Ser Val Glu Phe 385 390 395 400 Arg His Leu Gln Pro Lys Glu Met Lys Cys Ser Thr Gly Ser Lys Gly 405 410 415 Asn Glu Gly Cys His Met Val Thr Glu Glu Leu His Ser Ile Thr Phe 420 425 430 Glu Thr Gln Ile Cys Leu Tyr Gly Leu Thr Ile Asn Leu Glu Thr Ser 435 440 445 Ser Leu Pro Val Val Met Ile Ser Asn Val Ser Gln Leu Pro Asn Ala 450 455 460 Trp Ala Ser Ile Ile Trp Tyr Asn Val Ser Thr Asn Asp Ser Gln Asn 465 470 475 480 Leu Val Phe Phe Asn Asn Pro Pro Ser Val Thr Leu Gly Gln Leu Leu 485 490 495 Glu Val Met Ser Trp Gln Phe Ser Ser Tyr Val Gly Arg Gly Leu Asn 500 505 510 Ser Glu Gln Leu Asn Met Leu Ala Glu Lys Leu Thr Val Gln Ser Asn 515 520 525 Tyr Asn Asp Gly His Leu Thr Trp Ala Lys Phe Cys Lys Glu His Leu 530 535 540 Pro Gly Lys Thr Phe Thr Phe Trp Thr Trp Leu Glu Ala Ile Leu Asp 545 550 555 560 Leu Ile Lys Lys His Ile Leu Pro Leu Trp Ile Asp Gly Tyr Ile Met 565 570 575 Gly Phe Val Ser Lys Glu Lys Glu Arg Leu Leu Leu Lys Asp Lys Met 580 585 590 Pro Gly Thr Phe Leu Leu Arg Phe Ser Glu Ser His Leu Gly Gly Ile 595 600 605 Thr Phe Thr Trp Val Asp Gln Ser Glu Asn Gly Glu Val Arg Phe His 610 615 620 Ser Val Glu Pro Tyr Asn Lys Gly Arg Leu Ser Ala Leu Ala Phe Ala 625 630 635 640 Asp Ile Leu Arg Asp Tyr Lys Val Ile Met Ala Glu Asn Ile Pro Glu 645 650 655 Asn Pro Leu Lys Tyr Leu Tyr Pro Asp Ile Pro Lys Asp Lys Ala Phe 660 665 670 Gly Lys His Tyr Ser Ser Gln Pro Cys Glu Val Ser Arg Pro Thr Glu 675 680 685 Arg Gly Asp Lys Gly Tyr Val Pro Ser Val Phe Ile Pro Ile Ser Thr 690 695 700 Ile Arg Ser Asp Ser Thr Glu Pro Gln Ser Pro Ser Asp Leu Leu Pro 705 710 715 720 Met Ser Pro Ser Ala Tyr Ala Val Leu Arg Glu Asn Leu Ser Pro Thr 725 730 735 Thr Ile Glu Thr Ala Met Asn Ser Pro Tyr Ser Ala Glu 740 745 793 amino acids amino acid single linear protein NO not provided 10 Met Ala Gly Trp Ile Gln Ala Gln Gln Leu Gln Gly Asp Ala Leu Arg 1 5 10 15 Gln Met Gln Val Leu Tyr Gly Gln His Phe Pro Ile Glu Val Arg His 20 25 30 Tyr Leu Ala Gln Trp Ile Glu Ser Gln Pro Trp Asp Ala Ile Asp Leu 35 40 45 Asp Asn Pro Gln Asp Arg Gly Gln Ala Thr Gln Leu Leu Glu Gly Leu 50 55 60 Val Gln Glu Leu Gln Lys Lys Ala Glu His Gln Val Gly Glu Asp Gly 65 70 75 80 Phe Leu Leu Lys Ile Lys Leu Gly His Tyr Ala Thr Gln Leu Gln Asn 85 90 95 Thr Tyr Asp Arg Cys Pro Met Glu Leu Val Arg Cys Ile Arg His Ile 100 105 110 Leu Tyr Asn Glu Gln Arg Leu Val Arg Glu Ala Asn Asn Cys Ser Ser 115 120 125 Pro Ala Gly Val Leu Val Asp Ala Met Ser Gln Lys His Leu Gln Ile 130 135 140 Asn Gln Arg Phe Glu Glu Leu Arg Leu Ile Thr Gln Asp Thr Glu Asn 145 150 155 160 Glu Leu Lys Lys Leu Gln Gln Thr Gln Glu Tyr Phe Ile Ile Gln Tyr 165 170 175 Gln Glu Ser Leu Arg Ile Gln Ala Gln Phe Ala Gln Leu Gly Gln Leu 180 185 190 Asn Pro Gln Glu Arg Met Ser Arg Glu Thr Ala Leu Gln Gln Lys Gln 195 200 205 Val Ser Leu Glu Thr Trp Leu Gln Arg Glu Ala Gln Thr Leu Gln Gln 210 215 220 Tyr Arg Val Glu Leu Ala Glu Lys His Gln Lys Thr Leu Gln Leu Leu 225 230 235 240 Arg Lys Gln Gln Thr Ile Ile Leu Asp Asp Glu Leu Ile Gln Trp Lys 245 250 255 Arg Arg Gln Gln Leu Ala Gly Asn Gly Gly Pro Pro Glu Gly Ser Leu 260 265 270 Asp Val Leu Gln Ser Trp Cys Glu Lys Leu Ala Glu Ile Ile Trp Gln 275 280 285 Asn Arg Gln Gln Ile Arg Arg Ala Glu His Leu Cys Gln Gln Leu Pro 290 295 300 Ile Pro Gly Pro Val Glu Glu Met Leu Ala Glu Val Asn Ala Thr Ile 305 310 315 320 Thr Asp Ile Ile Ser Ala Leu Val Thr Ser Thr Phe Ile Ile Glu Lys 325 330 335 Gln Pro Pro Gln Val Leu Lys Thr Gln Thr Lys Phe Ala Ala Thr Val 340 345 350 Arg Leu Leu Val Gly Gly Lys Leu Asn Val His Met Asn Pro Pro Gln 355 360 365 Val Lys Ala Thr Ile Ile Ser Glu Gln Gln Ala Lys Ser Leu Leu Lys 370 375 380 Asn Glu Asn Thr Arg Asn Glu Cys Ser Gly Glu Ile Leu Asn Asn Cys 385 390 395 400 Cys Val Met Glu Tyr His Gln Ala Thr Gly Thr Leu Ser Ala His Phe 405 410 415 Arg Asn Met Ser Leu Lys Arg Ile Lys Arg Ala Asp Arg Arg Gly Ala 420 425 430 Glu Ser Val Thr Glu Glu Lys Phe Thr Val Leu Phe Glu Ser Gln Phe 435 440 445 Ser Val Gly Ser Asn Glu Leu Val Phe Gln Val Lys Thr Leu Ser Leu 450 455 460 Pro Val Val Val Ile Val His Gly Ser Gln Asp His Asn Ala Thr Ala 465 470 475 480 Thr Val Leu Trp Asp Asn Ala Phe Ala Glu Pro Gly Arg Val Pro Phe 485 490 495 Ala Val Pro Asp Lys Val Leu Trp Pro Gln Leu Cys Glu Ala Leu Asn 500 505 510 Met Lys Phe Lys Ala Glu Val Gln Ser Asn Arg Gly Leu Thr Lys Glu 515 520 525 Asn Leu Val Phe Leu Ala Gln Lys Leu Phe Asn Ile Ser Ser Asn His 530 535 540 Leu Glu Asp Tyr Asn Ser Met Ser Val Ser Trp Ser Gln Phe Asn Arg 545 550 555 560 Glu Asn Leu Pro Gly Trp Asn Tyr Thr Phe Trp Gln Trp Phe Asp Gly 565 570 575 Val Met Glu Val Leu Lys Lys His His Lys Pro His Trp Asn Asp Gly 580 585 590 Ala Ile Leu Gly Phe Val Asn Lys Gln Gln Ala His Asp Leu Leu Ile 595 600 605 Asn Lys Pro Asp Gly Thr Phe Leu Leu Arg Phe Ser Asp Ser Glu Ile 610 615 620 Gly Gly Ile Thr Ile Ala Trp Lys Phe Asp Ser Pro Asp Arg Asn Leu 625 630 635 640 Trp Asn Leu Lys Pro Phe Thr Thr Arg Asp Phe Ser Ile Arg Ser Leu 645 650 655 Ala Asp Arg Leu Gly Asp Leu Asn Tyr Leu Ile Tyr Val Phe Pro Asp 660 665 670 Arg Pro Lys Asp Glu Val Phe Ala Lys Tyr Tyr Thr Pro Val Leu Ala 675 680 685 Lys Ala Val Asp Gly Tyr Val Lys Pro Gln Ile Lys Gln Val Val Pro 690 695 700 Glu Phe Val Asn Ala Ser Thr Asp Ala Gly Ala Ser Ala Thr Tyr Met 705 710 715 720 Asp Gln Ala Pro Ser Pro Val Val Cys Pro Gln Pro His Tyr Asn Met 725 730 735 Tyr Pro Pro Asn Pro Asp Pro Val Leu Asp Gln Asp Gly Glu Phe Asp 740 745 750 Leu Asp Glu Ser Met Asp Val Ala Arg His Val Glu Glu Leu Leu Arg 755 760 765 Arg Pro Met Asp Ser Leu Asp Ala Arg Leu Ser Pro Pro Ala Gly Leu 770 775 780 Phe Thr Ser Ala Arg Ser Ser Leu Ser 785 790 786 amino acids amino acid single linear protein NO not provided 11 Met Ala Met Trp Ile Gln Ala Gln Gln Leu Gln Gly Asp Ala Leu His 1 5 10 15 Gln Met Gln Ala Leu Tyr Gly Gln His Phe Pro Ile Glu Val Arg His 20 25 30 Tyr Leu Ser Gln Trp Ile Glu Ser Gln Ala Trp Asp Ser Ile Asp Leu 35 40 45 Asp Asn Pro Gln Glu Asn Ile Lys Ala Thr Gln Leu Leu Glu Gly Leu 50 55 60 Val Gln Glu Leu Gln Lys Lys Ala Glu His Gln Val Gly Glu Asp Gly 65 70 75 80 Phe Leu Leu Lys Ile Lys Leu Gly His Tyr Ala Thr Gln Leu Gln Ser 85 90 95 Thr Tyr Asp Arg Cys Pro Met Glu Leu Val Arg Cys Ile Arg His Ile 100 105 110 Leu Tyr Asn Glu Gln Arg Leu Val Arg Glu Ala Asn Asn Gly Ser Ser 115 120 125 Pro Ala Gly Ser Leu Ala Asp Ala Met Ser Gln Lys His Leu Gln Ile 130 135 140 Asn Gln Thr Phe Glu Glu Leu Arg Leu Ile Thr Gln Asp Thr Glu Asn 145 150 155 160 Glu Leu Lys Lys Leu Gln Gln Thr Gln Glu Tyr Phe Ile Ile Gln Tyr 165 170 175 Gln Glu Ser Leu Arg Ile Gln Ala Gln Phe Ala Gln Leu Gly Gln Leu 180 185 190 Asn Pro Gln Glu Arg Met Ser Arg Glu Thr Ala Leu Gln Gln Lys Gln 195 200 205 Val Ser Leu Glu Thr Trp Leu Gln Arg Glu Ala Gln Thr Leu Gln Gln 210 215 220 Tyr Arg Val Glu Leu Ala Glu Lys His Gln Lys Thr Leu Gln Leu Leu 225 230 235 240 Arg Lys Gln Gln Thr Ile Ile Leu Asp Asp Glu Leu Ile Gln Trp Lys 245 250 255 Arg Arg Gln Gln Leu Ala Gly Asn Gly Gly Pro Pro Glu Gly Ser Leu 260 265 270 Asp Val Leu Gln Ser Trp Cys Glu Lys Leu Ala Glu Ile Ile Trp Gln 275 280 285 Asn Arg Gln Gln Ile Arg Arg Ala Glu His Leu Cys Gln Gln Leu Pro 290 295 300 Ile Pro Gly Pro Val Glu Glu Met Leu Ala Glu Val Asn Ala Thr Ile 305 310 315 320 Thr Asp Ile Ile Ser Ala Leu Val Thr Ser Thr Phe Ile Ile Glu Lys 325 330 335 Gln Pro Pro Gln Val Leu Lys Thr Gln Thr Lys Phe Ala Ala Thr Val 340 345 350 Arg Leu Leu Val Gly Gly Lys Leu Asn Val His Met Asn Pro Pro Gln 355 360 365 Val Lys Ala Thr Ile Ile Ser Glu Gln Gln Ala Lys Ser Leu Leu Lys 370 375 380 Asn Glu Asn Thr Arg Asn Asp Tyr Ser Gly Glu Ile Leu Asn Asn Cys 385 390 395 400 Cys Val Met Glu Tyr His Gln Ala Thr Gly Thr Leu Ser Ala His Phe 405 410 415 Arg Asn Met Ser Leu Lys Arg Ile Lys Arg Ser Asp Arg Arg Gly Ala 420 425 430 Gly Ser Val Thr Glu Glu Lys Phe Thr Ile Leu Phe Asp Ser Gln Phe 435 440 445 Ser Val Gly Gly Asn Glu Leu Val Phe Gln Val Lys Thr Leu Ser Leu 450 455 460 Pro Val Val Val Ile Val His Gly Ser Gln Asp Asn Asn Ala Thr Ala 465 470 475 480 Thr Val Leu Trp Asp Asn Ala Phe Ala Glu Pro Gly Arg Val Pro Phe 485 490 495 Ala Val Pro Asp Lys Val Leu Trp Pro Gln Leu Cys Glu Ala Leu Asn 500 505 510 Met Lys Phe Lys Ala Glu Val Gln Ser Asn Arg Gly Leu Thr Lys Glu 515 520 525 Asn Leu Val Phe Leu Ala Gln Lys Leu Phe Asn Ile Ser Ser Asn His 530 535 540 Leu Glu Asp Tyr Asn Ser Met Ser Val Ser Trp Ser Gln Phe Asn Arg 545 550 555 560 Glu Asn Leu Pro Gly Arg Asn Tyr Thr Phe Trp Gln Trp Phe Asp Gly 565 570 575 Val Met Glu Val Leu Lys Lys His Leu Lys Pro His Trp Asn Asp Gly 580 585 590 Ala Ile Leu Gly Phe Val Asn Lys Gln Gln Ala His Asp Leu Leu Ile 595 600 605 Asn Lys Pro Asp Gly Thr Phe Leu Leu Arg Phe Ser Asp Ser Glu Ile 610 615 620 Gly Gly Ile Thr Ile Ala Trp Lys Phe Asp Ser Gln Glu Arg Met Phe 625 630 635 640 Trp Asn Leu Met Pro Phe Thr Thr Arg Asp Phe Ser Ile Arg Ser Leu 645 650 655 Ala Asp Arg Leu Gly Asp Leu Asn Tyr Leu Ile Tyr Val Phe Pro Asp 660 665 670 Arg Pro Lys Asp Glu Val Tyr Ser Lys Tyr Tyr Thr Pro Val Pro Cys 675 680 685 Glu Pro Ala Thr Ala Lys Ala Ala Asp Gly Tyr Val Lys Pro Gln Ile 690 695 700 Lys Gln Val Val Pro Glu Phe Ala Asn Ala Ser Thr Asp Ala Gly Ser 705 710 715 720 Gly Ala Thr Tyr Met Asp Gln Ala Pro Ser Pro Val Val Cys Pro Gln 725 730 735 Ala His Tyr Asn Met Tyr Pro Pro Asn Pro Asp Ser Val Leu Asp Thr 740 745 750 Asp Gly Asp Phe Asp Leu Glu Asp Thr Met Asp Val Ala Arg Arg Val 755 760 765 Glu Glu Leu Leu Gly Arg Pro Met Asp Ser Gln Trp Ile Pro His Ala 770 775 780 Gln Ser 785 837 amino acids amino acid single linear protein NO not provided 12 Met Ser Leu Trp Gly Leu Ile Ser Lys Met Ser Pro Glu Lys Leu Gln 1 5 10 15 Arg Leu Tyr Val Asp Phe Pro Gln Arg Leu Arg His Leu Leu Ala Asp 20 25 30 Trp Leu Glu Ser Gln Pro Trp Glu Phe Leu Val Gly Ser Asp Ala Phe 35 40 45 Cys Tyr Asn Met Ala Ser Ala Leu Leu Ser Ala Thr Val Gln Arg Leu 50 55 60 Gln Ala Thr Ala Gly Glu Gln Gly Lys Gly Asn Ser Ile Leu Pro His 65 70 75 80 Ile Ser Thr Leu Glu Ser Ile Tyr Gln Arg Asp Pro Leu Lys Leu Val 85 90 95 Ala Thr Ile Arg Gln Ile Leu Gln Gly Glu Lys Lys Ala Val Ile Glu 100 105 110 Glu Phe Arg His Leu Pro Gly Pro Phe His Arg Lys Gln Glu Glu Leu 115 120 125 Lys Phe Thr Thr Pro Leu Gly Arg Leu His His Arg Val Arg Glu Thr 130 135 140 Arg Leu Leu Arg Glu Ser Leu His Leu Gly Pro Lys Thr Gly Gln Val 145 150 155 160 Ser Leu Gln Asn Leu Ile Asp Pro Pro Leu Asn Gly Pro Gly Pro Ser 165 170 175 Glu Asp Leu Pro Thr Ile Leu Gln Gly Thr Val Gly Asp Leu Glu Thr 180 185 190 Thr Gln Pro Leu Val Leu Leu Arg Ile Gln Ile Trp Lys Arg Gln Gln 195 200 205 Gln Leu Ala Gly Asn Gly Thr Pro Phe Glu Glu Ser Leu Ala Gly Leu 210 215 220 Gln Glu Arg Cys Glu Ser Leu Val Glu Ile Tyr Ser Gln Leu His Gln 225 230 235 240 Glu Ile Gly Ala Ala Ser Gly Glu Leu Glu Pro Lys Thr Arg Ala Ser 245 250 255 Leu Ile Ser Arg Leu Asp Glu Val Leu Arg Thr Leu Val Thr Ser Ser 260 265 270 Phe Leu Val Glu Lys Gln Pro Pro Gln Val Leu Lys Thr Gln Thr Lys 275 280 285 Phe Gln Ala Gly Val Arg Phe Leu Leu Gly Leu Gln Phe Leu Gly Thr 290 295 300 Ser Thr Lys Pro Pro Met Val Arg Ala Asp Met Val Thr Glu Lys Gln 305 310 315 320 Ala Arg Glu Leu Ser Leu Ser Gln Gly Pro Gly Thr Gly Val Glu Ser 325 330 335 Thr Gly Glu Ile Met Asn Asn Thr Val Pro Leu Glu Asn Ser Ile Pro 340 345 350 Ser Asn Cys Cys Ser Ala Leu Phe Lys Asn Leu Leu Leu Lys Lys Ile 355 360 365 Lys Arg Cys Glu Arg Lys Gly Thr Glu Ser Val Thr Glu Glu Lys Cys 370 375 380 Ala Val Leu Phe Ser Thr Ser Phe Thr Leu Gly Pro Asn Lys Leu Leu 385 390 395 400 Ile Gln Leu Gln Ala Leu Ser Leu Ser Leu Val Val Ile Val His Gly 405 410 415 Asn Gln Asp Asn Asn Ala Lys Ala Thr Ile Leu Trp Asp Asn Ala Phe 420 425 430 Ser Glu Met Asp Arg Val Pro Phe Val Val Gly Glu Arg Val Pro Trp 435 440 445 Glu Lys Met Cys Glu Thr Leu Asn Leu Lys Phe Met Val Glu Val Gly 450 455 460 Thr Ser Arg Gly Leu Leu Pro Glu His Phe Leu Phe Leu Ala Gln Lys 465 470 475 480 Ile Phe Asn Asp Asn Ser Leu Ser Val Glu Ala Phe Gln His Arg Cys 485 490 495 Val Ser Trp Ser Gln Phe Asn Lys Glu Ile Leu Leu Gly Arg Gly Phe 500 505 510 Thr Phe Trp Gln Trp Phe Asp Gly Val Leu Asp Leu Thr Lys Arg Cys 515 520 525 Leu Arg Ser Tyr Trp Ser Asp Arg Leu Ile Ile Gly Phe Ile Ser Lys 530 535 540 Gln Tyr Val Thr Ser Leu Leu Leu Asn Glu Pro Asp Gly Thr Phe Leu 545 550 555 560 Leu Arg Phe Ser Asp Ser Glu Ile Gly Gly Ile Thr Ile Ala His Val 565 570 575 Ile Arg Gly Gln Asp Gly Ser Ser Gln Ile Glu Asn Ile Gln Pro Phe 580 585 590 Ser Ala Lys Asp Leu Ser Ile Arg Ser Leu Gly Asp Arg Ile Arg Asp 595 600 605 Leu Ala Gln Leu Lys Asn Leu Tyr Pro Lys Lys Pro Lys Asp Glu Ala 610 615 620 Phe Arg Ser His Tyr Lys Pro Glu Gln Met Gly Lys Asp Gly Arg Gly 625 630 635 640 Tyr Val Ser Thr Thr Ile Lys Met Thr Val Glu Arg Asp Gln Pro Leu 645 650 655 Pro Thr Pro Glu Pro Gln Met Pro Ala Met Val Pro Pro Tyr Asp Leu 660 665 670 Gly Met Ala Pro Asp Ala Ser Met Gln Leu Ser Ser Asp Met Gly Tyr 675 680 685 Pro Pro Gln Ser Ile His Ser Phe Gln Ser Leu Glu Glu Ser Met Ser 690 695 700 Val Leu Pro Ser Phe Gln Glu Pro His Leu Gln Met Pro Pro Asn Met 705 710 715 720 Ser Gln Ile Thr Met Pro Phe Asp Gln Pro His Pro Gln Gly Leu Leu 725 730 735 Gln Cys Gln Ser Gln Glu His Ala Val Ser Ser Pro Glu Pro Met Leu 740 745 750 Trp Ser Asp Val Thr Met Val Glu Asp Ser Cys Leu Thr Gln Pro Val 755 760 765 Gly Gly Phe Pro Gln Gly Thr Trp Val Ser Glu Asp Met Tyr Pro Pro 770 775 780 Leu Leu Pro Pro Thr Glu Gln Asp Leu Thr Lys Leu Leu Leu Glu Asn 785 790 795 800 Gln Gly Glu Gly Gly Gly Ser Leu Gly Ser Gln Pro Leu Leu Lys Pro 805 810 815 Ser Pro Tyr Gly Gln Ser Gly Ile Ser Leu Ser His Leu Asp Leu Arg 820 825 830 Thr Asn Pro Ser Trp 835 761 amino acids amino acid single linear protein NO not provided 13 Met Ser Leu Trp Lys Arg Ile Ser Ser His Val Asp Cys Glu Gln Arg 1 5 10 15 Met Ala Ala Tyr Tyr Glu Glu Lys Gly Met Leu Glu Leu Arg Leu Cys 20 25 30 Leu Ala Pro Trp Ile Glu Asp Arg Ile Met Ser Glu Gln Ile Thr Pro 35 40 45 Asn Thr Thr Asp Gln Leu Glu Arg Val Ala Leu Lys Phe Asn Glu Asp 50 55 60 Leu Gln Gln Lys Leu Leu Ser Thr Arg Thr Ala Ser Asp Gln Ala Leu 65 70 75 80 Lys Phe Arg Val Val Glu Leu Cys Ala Leu Ile Gln Arg Ile Ser Ala 85 90 95 Val Glu Leu Tyr Thr His Leu Arg Ser Gly Leu Gln Lys Glu Leu Gln 100 105 110 Leu Val Thr Glu Lys Ser Val Ala Ala Thr Ala Gly Gln Ser Met Pro 115 120 125 Leu Asn Pro Tyr Asn Met Asn Asn Thr Pro Met Val Thr Gly Tyr Met 130 135 140 Val Asp Pro Ser Asp Leu Leu Ala Val Ser Asn Ser Cys Asn Pro Pro 145 150 155 160 Val Val Gln Gly Ile Gly Pro Ile His Asn Val Gln Asn Thr Gly Ile 165 170 175 Ala Ser Pro Ala Leu Gly Met Val Thr Pro Lys Val Glu Leu Tyr Glu 180 185 190 Val Gln His Gln Ile Met Gln Ser Leu Asn Glu Phe Gly Asn Cys Ala 195 200 205 Asn Ala Leu Lys Leu Leu Ala Gln Asn Tyr Ser Tyr Met Leu Asn Ser 210 215 220 Thr Ser Ser Pro Asn Ala Glu Ala Ala Tyr Arg Ser Leu Ile Asp Glu 225 230 235 240 Lys Ala Ala Ile Val Leu Thr Met Arg Arg Ser Phe Met Tyr Tyr Glu 245 250 255 Ser Leu His Glu Met Val Ile His Glu Leu Lys Asn Trp Arg His Gln 260 265 270 Gln Ala Gln Ala Gly Asn Gly Ala Pro Phe Asn Glu Gly Ser Leu Asp 275 280 285 Asp Ile Gln Arg Cys Phe Glu Met Leu Glu Ser Phe Ile Ala His Met 290 295 300 Leu Ala Ala Val Lys Glu Leu Met Arg Val Arg Leu Val Thr Glu Glu 305 310 315 320 Pro Glu Leu Thr His Leu Leu Glu Gln Val Gln Asn Ala Gln Lys Asn 325 330 335 Leu Val Cys Ser Ala Phe Ile Val Asp Lys Gln Pro Pro Gln Val Met 340 345 350 Lys Thr Asn Thr Arg Phe Ala Ala Ser Val Arg Trp Leu Ile Gly Ser 355 360 365 Gln Leu Gly Ile His Asn Asn Pro Pro Thr Val Glu Cys Ile Ile Met 370 375 380 Ser Glu Ile Gln Ser Gln Arg Phe Val Thr Arg Asn Thr Gln Met Asp 385 390 395 400 Asn Ser Ser Leu Ser Gly Gln Ser Ser Gly Glu Ile Gln Asn Ala Ser 405 410 415 Ser Thr Met Glu Tyr Gln Gln Asn Asn His Val Phe Ser Ala Ser Phe 420 425 430 Arg Asn Met Gln Leu Lys Lys Ile Lys Arg Ala Glu Lys Lys Gly Thr 435 440 445 Glu Ser Val Met Asp Glu Lys Phe Ala Leu Phe Phe Tyr Thr Thr Thr 450 455 460 Thr Val Asn Asp Phe Gln Ile Arg Val Trp Thr Leu Ser Leu Pro Val 465 470 475 480 Val Val Ile Val His Gly Asn Gln Glu Pro Gln Ser Trp Ala Thr Ile 485 490 495 Thr Trp Asp Asn Ala Phe Ala Glu Ile Val Arg Asp Pro Phe Met Ile 500 505 510 Thr Asp Arg Val Thr Trp Ala Gln Leu Ser Val Ala Leu Asn Ile Lys 515 520 525 Phe Gly Ser Cys Thr Gly Arg Ser Leu Thr Ile Asp Asn Leu Asp Phe 530 535 540 Leu Tyr Glu Lys Leu Gln Arg Glu Glu Arg Ser Glu Tyr Ile Thr Trp 545 550 555 560 Asn Gln Phe Cys Lys Glu Pro Met Pro Asp Arg Ser Phe Thr Phe Trp 565 570 575 Glu Trp Phe Phe Ala Ile Met Lys Leu Thr Lys Asp His Met Leu Gly 580 585 590 Met Trp Lys Ala Gly Cys Ile Met Gly Phe Ile Asn Lys Thr Lys Ala 595 600 605 Gln Thr Asp Leu Leu Arg Ser Val Tyr Gly Ile Gly Thr Phe Leu Leu 610 615 620 Arg Phe Ser Asp Ser Glu Leu Gly Gly Val Thr Ile Ala Tyr Val Asn 625 630 635 640 Glu Asn Gly Leu Val Thr Met Leu Ala Pro Trp Thr Ala Arg Asp Phe 645 650 655 Gln Val Leu Asn Leu Ala Asp Arg Ile Arg Asp Leu Asp Val Leu Cys 660 665 670 Trp Leu His Pro Ser Asp Arg Asn Ala Ser Pro Val Lys Arg Asp Val 675 680 685 Ala Phe Gly Glu Phe Tyr Ser Lys Arg Gln Glu Pro Glu Pro Leu Val 690 695 700 Leu Asp Pro Val Thr Gly Tyr Val Lys Ser Thr Leu His Val His Val 705 710 715 720 Cys Arg Asn Gly Glu Asn Gly Ser Thr Ser Gly Thr Pro His His Ala 725 730 735 Gln Glu Ser Met Gln Leu Gly Asn Gly Asp Phe Gly Met Ala Asp Phe 740 745 750 Asp Thr Ile Thr Asn Phe Glu Asn Phe 755 760 

What is claimed is:
 1. A method of identifying a compound that binds to the N-terminal domain of a Signal Transducer And Activator of Transcription (STAT) comprising: (a) obtaining a set of atomic coordinates defining the three-dimensional structure of a crystal of an N-terminal fragment of a STAT that effectively diffracts X-rays for the determination of the atomic coordinates of the N-terminal fragment to a resolution of better than 5.0 Angstroms; wherein the N-terminal fragment of the STAT protein consists of 122 to 128 amino acid residues and comprises a peptide consisting of amino acid residues 2-123 of SEQ ID NO:9; and wherein said crystal has a space group of P6₅22 with unit cell dimensions of a=79.51 Å, b=79.51 Å, and c=84.68 Å; (b) selecting a compound by performing rational drug design with the atomic coordinates obtained in step (a), wherein said selecting is performed in conjunction with computer modeling; (c) contacting the compound with a dimeric STAT protein N-terninal domain; and (d) detecting the binding of the compound with the N-terminal domain; wherein a compound is selected that binds to the N-terminal domain.
 2. The method of claim 1 wherein contacting the compound with a dimeric STAT protein N-terminal domain is performed with a STAT protein fragment containing a STAT protein N-terminal domain comprising SEQ ID NO:1.
 3. The method of claim 2 wherein the STAT protein fragment is labeled.
 4. The method of claim 2 wherein the STAT protein fragment is bound to a solid support.
 5. The method of claim 1 wherein said peptide further comprises SEQ ID NO:3; and wherein SEQ ID NO:3 is covalently attached to the N-terminal amino acid of said peptide.
 6. The method of claim 5 wherein said peptide further comprises amino acid residue 124 of SEQ ID NO:9.
 7. The crystal of claim 1 wherein said peptide further comprises amino acid residue 124 of SEQ ID NO:9.
 8. A method of identifying a compound that either enhances or diminishes the binding of the dimeric Signal Transducer And Activator of Transcription (STAT) proteins to each other and/or their nucleic acid binding site comprising: (a) obtaining a set of atomic coordinates defining the three-dimensional structure of a crystal of an N-terminal fragment of a STAT that effectively diffracts X-rays for the determination of the atomic coordinates of the N-terminal fragment to a resolution of better than 5.0 Angstroms; wherein the N-tenninal fragment of the STAT protein consists of 122 to 128 amino acid residues and comprises a peptide consisting of amino acid residues 2-123 of SEQ ID NO:9; and wherein said crystal has a space group of P6₅22 with unit cell dimensions of a=79.51 Å, b=79.51 Å, and c=84.68 Å: (b) selecting a compound by performing rational drug design with the atomic coordinates obtained in step (a), wherein said selecting is performed in conjunction with computer modeling; (c) contacting the compound with two or more dimeric STAT proteins in the presence of a nucleic acid containing at least two adjacent weak binding sites for STAT protein dimers; and (d) detecting the effect of the compound on the binding of the dimeric STAT proteins to each other and/or to the nucleic acid; wherein the compound is selected when it either enhances or diminishes the binding of the dimeric STAT proteins to each other and/or the nucleic acid.
 9. The method of claim 8 wherein said peptide further comprises amino acid residue 124 of SEQ ID NO:9 and SEQ ID NO:3; and wherein SEQ ID NO:3 is covalently attached to the N-terminal amino acid of said peptide.
 10. A method for identifying a compound that enhances or diminishes the ability of Signal Transducer And Activator of Transcription (STAT) protein dimers to induce the expression of a gene operably under the control of a promoter containing at least two adjacent weak binding sites for STAT protein dimers comprising: (a) obtaining a set of atomic coordinates defining the three-dimensional structure of a crystal of an N-terminal fragment of a STAT that effectively diffracts X-rays for the determination of the atomic coordinates of the N-terminal fragment to a resolution of better than 5.0 Angstroms; wherein the N-terminal fragment of the STAT protein consists of 122 to 128 amino acid residues and comprises a peptide consisting of amino acid residues 2-123 of SEQ ID NO:9; and wherein said crystal has a space group of P6₅22 with unit cell dimensions of a=79.51 Å, b=79.51 Å, and c=84.68 Å; (b) selecting a compound by performing rational drug design with the atomic coordinates obtained in step (a), wherein said selecting is performed in conjunction with computer modeling; (c) measuring the level of expression of a first reporter gene and a second reporter gene contained by a host cell in the presence and absence of the compound; wherein the first reporter gene is operably linked to a first promoter containing at least two adjacent weak binding sites for STAT protein dimers, and the second reporter gene is operably linked to a second promoter comprising at least one strong binding site for a STAT protein dimer; wherein the binding of STAT protein dimers to the two adjacent weak binding sites induces the expression of the first reporter gene, and wherein the binding of the STAT protein dimer to the strong binding site induces the expression of the second reporter gene; and wherein the host cell contains STAT protein dimers; and (d) comparing the level of expression of the first reporter gene with that of the second reporter gene in the presence and absence of the compound, wherein when the presence of the compound results in an increase in the level of expression of the first reporter gene but not that of the second reporter gene, the compound is identified as a compound that enhances the ability of STAT protein dimers to induce the expression of a gene operably under the control of a promoter containing at least two adjacent weak binding sites for STAT protein dimers; and when the presence of the compound results in a decrease in the level of expression of the first reporter gene but not that of the second reporter gene the compound is identified as a compound that inhibits the ability of STAT protein dimers to induce the expression of a gene operably under the control of a promoter containing at least two adjacent weak binding sites for STAT protein dimers.
 11. The method of claim 10 wherein the host cell is a mammalian cell.
 12. The method of claim 10 wherein the first reporter gene is contained by a first host cell, and the second reporter gene is contained by a second host cell; and wherein the first host cell and second host cell both contain STAT protein dimers.
 13. The method of claim 10 wherein the weak STAT binding sites are selected from the group consisting of sites present in the regulatory regions of the MIG gene, the c-fos gene and the interferon-γ gene.
 14. The method of claim 10 wherein said peptide further comprises SEQ ID NO:3; and wherein SEQ ID NO:3 is covalently attached to the N-terminal amino acid of said peptide.
 15. The method of claim 14 wherein said peptide further comprises amino acid residue 124 of SEQ ID NO:9.
 16. The crystal of claim 10 wherein said peptide further comprises amino acid residue 124 of SEQ ID NO:9. 